string: Reorder includes to avoid compiler warning
string: Reorder includes to avoid compiler warning

This avoids warnings like this on macOS:

In file included from usual/string.c:23:
./usual/ctype.h:65:17: warning: 'safe_isascii' is static but used in inline function '__tolower_l' which is not static

markokr/libusual,markokr/libusual,markokr/libusual,markokr/libusual