Suppress -Wunused-function

[k0kubun]

I saw a compiler warning when compiling ruby/ruby. This PR attempts to suppress it by making it an inline function, which is usually not warned by -Wunused-function.

In file included from ../../../../ext/json/parser/parser.c:23:
../../../../ext/json/parser/../simd/simd.h:186:28: warning: ‘find_simd_implementation’ defined but not used [-Wunused-function]
  186 | static SIMD_Implementation find_simd_implementation(void) {
      |                            ^~~~~~~~~~~~~~~~~~~~~~~~

I'm not sure why it's happening (or HAVE_SIMD is not defined) on my ruby build, but it does seem unused in parser.c when HAVE_SIMD is not defined. So it does seem like a legit warning in some cases.

[nobu]

That warning is due to simd/conf.rb was required and evaluated only once, as described at #819.