fix variable name

PKG_SITES is correct now

toolchain/gmp/Makefile.inc

@@ -7,6 +7,6 @@ PKG_NAME:=		gmp
 PKG_VERSION:=		4.3.1
 PKG_RELEASE:=		1
 PKG_MD5SUM:=		26cec15a90885042dd4a15c4003b08ae
-MASTER_SITES:=		${MASTER_SITE_GNU:=gmp/}
+PKG_SITES:=		${MASTER_SITE_GNU:=gmp/}
 DISTFILES:=		$(PKG_NAME)-$(PKG_VERSION).tar.bz2
 GMP_BUILD_DIR:=		$(TOOLCHAIN_BUILD_DIR)/$(PKG_NAME)-$(PKG_VERSION)