[201911] [FRR] Upgrade FRR to frr-7.2.1-s4 tag (sonic-net#8572)

Why I did it
Update FRR 7.2.1 head. The following is a list of new commits.

5ae667a1f Merge pull request sonic-net#9335 from FRRouting/mergify/bp/stable/7.2/pr-9214
eb679e8a1 zebra: bugfix of error quit of zebra, due to no nexthop ACTIVE
80d2eaa98 Merge pull request sonic-net#8886 from FRRouting/mergify/bp/stable/7.2/pr-8876
1eeab2c1e lib: remove pure attribute from functions that modify memory
eb00dc4ec Merge pull request sonic-net#6944 from LabNConsulting/working/lb/7.2/valgrind-supp-libyang
b9d6d05bf bgpd: suppress new libyang_1.0 related loss reports
8c26a71eb Merge pull request sonic-net#6562 from ton31337/fix/configuration_for_labeled_unicast_in_place_7.2
386a1719c bgpd: Make sure network/aggregate-address commands lay down under labeled safi
b01c8bf28 Merge pull request sonic-net#6526 from ton31337/fix/set_ipv6_ll_if_global_zero_7.2
c382833e8 bgpd: Use IPv6 LL address as nexthop if global was set to ::/LL
99509b835 Merge pull request sonic-net#6395 from opensourcerouting/7.2/init-config-perms
7eef8f7b1 build: use configfile mode in init script
4cbe07705 Merge pull request sonic-net#6360 from opensourcerouting/7.2/fix-warnings
84bb11785 nhrpd: clean up SA warning
aac726476 nhrpd: be more careful with linked lists
3a4b6d654 debian: Fix spelling error
756c67c6c Merge pull request sonic-net#6284 from opensourcerouting/7.2/gcc-10
65a116a64 Merge pull request sonic-net#6354 from ton31337/fix/communities_bgpd_crash_7.2
f7a00fd67 bgpd: Check to ensure community attributes exist before freeing them
a960f99c2 vrrpd: fix build on Fedora Rawhide
d4caff99f babeld: GCC complaining about no return in non-void function
a014c27ae babeld: fix build on Fedora Rawhide
79ff55b5b bgpd: remove unused variable
ff343e588 pimd: Make frr able to be built by gcc 10
9a3cf1ba2 ldpd: remove multiple definitions of thread_master
a19515bfe ldpd: fix another linking issue with GCC-10
b4c8de38c tests: fix build with GCC 10
4f27e8c85 ldpd: Fix linking error on Fedora Rawhide with GCC 10

How I did it
Update FRR 7.2 pointer and create a tag frr-7.2.1-s4.

rules/frr.mk

@@ -3,7 +3,7 @@
 FRR_VERSION = 7.2.1
 FRR_SUBVERSION = 0
 FRR_BRANCH = frr/7.2
-FRR_TAG = frr-7.2.1-s3
+FRR_TAG = frr-7.2.1-s4
 export FRR_VERSION FRR_SUBVERSION FRR_BRANCH FRR_TAG