gtypes.h: Make G_MININTn literals negative
gtypes.h: Make G_MININTn literals negative

This is more friendly to the GIR scanner; with previous definitions,
the constant values end up out of range for their stated integer type.

https://bugzilla.gnome.org/show_bug.cgi?id=756550

Distrotech/glib,johne53/MB3Glib,ieei/glib,endlessm/glib,MathieuDuponchelle/glib,MathieuDuponchelle/glib,endlessm/glib,johne53/MB3Glib,Distrotech/glib,mzabaluev/glib,Distrotech/glib,MathieuDuponchelle/glib,mzabaluev/glib,mzabaluev/glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,MathieuDuponchelle/glib,MathieuDuponchelle/glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,mzabaluev/glib,johne53/MB3Glib,mzabaluev/glib,ieei/glib,Distrotech/glib,ieei/glib,ieei/glib,Distrotech/glib,ieei/glib