Fix compiler warning in bigmstrcmp().

Fix compiler warning in bigmstrcmp().

Commit 86d78ef in PostgreSQL caused the compiler to report
the following warning when compiling pg_bigm on PostgreSQL 12
or later. This commit fixes it.

    warning: inline function 'bigmstrcmp' declared but never defined [enabled by default]

To suppress the compiler warning, this commit gets rid of
"extern inline" declaration for bigmstrcmp() and moves its definition
into the header file as "static inline" function.

Back-patch to pg_bigm 1.0. Since pg_bigm 1.1 and 1.0 don't support
PostgreSQL 12 or later, basically this commit doesn't need
to be back-patch to 1.1 and 1.0. But we do that because making
the code look similar in all branches for the ease of future
maintenance.

Author: Torikoshi Atsushi
Reviewed-by: Fujii Masao
Discussion: https://osdn.net/projects/pgbigm/lists/archive/hackers/2020-February/000327.html