buildme.sh: Add check for extern.h

[wryMitts]

On Alpine/musl, the buildme.sh script could benefit from a check of extern.h:

mv Scan.xsc Scan.c
cc -c  -Iinclude -Isrc -D_REENTRANT -D_GNU_SOURCE -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -Wdeclaration-after-statement -Os -fstack-clash-protection -Wformat -Werror=format-security -fno-plt -O2   -DVERSION=\"1.09\" -DXS_VERSION=\"1.09\" -fPIC "-I/usr/lib/perl5/core_perl/CORE"   Scan.c
Scan.xs:1:10: fatal error: EXTERN.h: No such file or directory
    1 | #include "EXTERN.h"
      |          ^~~~~~~~~~
compilation terminated.

This is resolved by running apk add perl-dev
https://pkgs.alpinelinux.org/package/edge/main/x86/perl-dev