Add missing <stdlib.h> include. Add ifdef so we compile without warnings

Add missing <stdlib.h> include. Add ifdef so we compile without warnings

	* gdk-pixbuf-csource.c: Add missing <stdlib.h> include.
	* io-png.c: (setup_png_transformations): Add ifdef so we compile
	without warnings with G_DISABLE_CHECKS on.
	* io-pnm.c: (gdk_pixbuf__pnm_image_load_increment): Add a missing
	const.
	* io-wbmp.c: (getin), (get_mbi): Add a missing const.
	* io-xbm.c: (gdk_pixbuf__xbm_image_load_real): Get rid of some
	unused locals and add an initial value to quiet the compiler's
	unintialized variable warning.
	* pixops/pixops.c: Put an ifdef around some dead code.