gthread-impl.c: add missing #include "config.h"

gthread-impl.c: add missing #include "config.h"

One of the features of our new symbol exporting regime is that it forces
config.h to be included at the top of every .c file.

This was missed in gthread-impl.c.

The only reason this wasn't noticed is because nothing in this file is
being used anymore.  We need those symbols to continue to be exported
for reasons of ABI backwards compatibility, however.