site: Add common-darwin

We need this to ensure there are some correct values when building with
meta-darwin. This matches the addition of a common-mingw site file in
the core.

(From OE-Core rev: 8c2ade6f48dcc522c68f798286d1c43bd2c1c8e7)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/site/common-darwin

@@ -0,0 +1,2 @@
+ac_cv_func_posix_getpwuid_r=${ac_cv_func_posix_getpwuid_r=yes}
+ac_cv_func_posix_getpwgid_r=${ac_cv_func_posix_getgrgid_r=yes}