Fix build failure under GCC 10.2

[felixonmars]

No description provided.

[damon-kwok]

The same error also occurs in GCC8.3. This PR can indeed solve the problem.

[ 31%] Linking CXX executable ../../../release/libponyrt.tests
[ 31%] Building C object src/libponyc/CMakeFiles/libponyc.dir/expr/reference.c.o
In file included from /home/damon/.ome_local/repo/ponyc-git/src/libponyc/expr/reference.c:18:
/home/damon/.ome_local/repo/ponyc-git/src/libponyc/expr/reference.c: In function ‘expr_contained_in_recover’:
/home/damon/.ome_local/repo/ponyc-git/src/libponyc/../common/ponyassert.h:16:23: error: this statement may fall through [-Werror=implicit-fallthrough=]
     ((expr) ? (void)0 : \
     ~~~~~~~~~~~~~~~~~~^~~
       ponyint_assert_fail(#expr, __FILE__, __LINE__, __func__))
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/damon/.ome_local/repo/ponyc-git/src/libponyc/expr/reference.c:222:9: note: in expansion of macro ‘pony_assert’
         pony_assert(0);
         ^~~~~~~~~~~
/home/damon/.ome_local/repo/ponyc-git/src/libponyc/expr/reference.c:224:7: note: here
       case TK_THIS:
       ^~~~
gmake[3]: warning:  Clock skew detected.  Your build may be incomplete.

[SeanTAllen]

Thanks @felixonmars