Older compilers do not yet have fmaintrin.h. This fixes compile errors:
```
x86_functions_fma.cc:51:39: error: '_mm_fmadd_ss' was not declared in this scope
x86_functions_fma.cc:85:35: error: '_mm256_fmadd_ps' was not declared in this scope
x86_functions_fma.cc:106:35: error: '_mm256_fmadd_ps' was not declared in this scope
x86_functions_fma.cc:129:39: error: '_mm_fmadd_ss' was not declared in this scope
```