common: include config.h in colorprocs.h to get GVDLL in autotools builds

common: include config.h in colorprocs.h to get GVDLL in autotools builds

An upcoming commit that corrects the storage-class attribute
declarations in colorprocs.h would otherwise cause this error:

/usr/lib/gcc/x86_64-pc-cygwin/10/../../../../x86_64-pc-cygwin/bin/ld: ../../lib/sparse/.libs/libsparse_C.a(DotIO.o): in function `Import_coord_clusters_from_dot':
/home/magja/graphviz/lib/sparse/DotIO.c:577: undefined reference to `colorxlate'

Towards https://gitlab.com/graphviz/graphviz/-/issues/2058.