Fix CI regression caused by 0934bcf1

Previously, the CI was failing to build because it didn't
see `locale_t` inside of `<string.h>`. This commit adds
a hacky fix to this so that CI stops failing.

For now, this pollutes the C namespace by including locale
functions and definitions inside of `<string.h>`. In the
future, we will come up with a mechanism to export `locale_t`
by itself.

src/header/string/cbindgen.toml

@@ -1,4 +1,4 @@
-sys_includes = ["stddef.h", "stdint.h", "strings.h"]
+sys_includes = ["stddef.h", "stdint.h", "strings.h", "locale.h"]
 include_guard = "_RELIBC_STRING_H"
 language = "C"
 style = "Tag"