meson: Use global function in argp check

meson: Use global function in argp check

clang is optimizing away parse_opt() since its marked static and not
used in main(), make life a bit harder for clang :)
removing static now emits the global symbols to be resolved during link
and when libargp is missing the error is thrown rightly

riscv64-yoe-linux-musl-ld: /tmp/a-5ba039.o: in function `.Lpcrel_hi0':
a.c:(.text+0x44): undefined reference to `argp_state_help'
clang-16: error: linker command failed with exit code 1 (use -v to see invocation)

Signed-off-by: Khem Raj <raj.khem@gmail.com>