glibc: fix applying of patches

Commit f4d9d7a removed support for version specific patches; but it also
broke applying of patches in the patches glibc dir.
Fix the issue by setting PATCH_DIR to $(PATH_PREFIX)/patches similar as
musl/common.mk.

Signed-off-by: Xinxing Hu <xinxing.huchn@gmail.com>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

toolchain/glibc/common.mk

@@ -16,6 +16,7 @@ PKG_HASH:=e54e0a934cd2bc94429be79da5e9385898d2306b9eaf3c92d5a77af96190f6bd
 PKG_SOURCE_SUBDIR:=$(PKG_NAME)-$(PKG_VERSION)
 HOST_BUILD_DIR:=$(BUILD_DIR_TOOLCHAIN)/$(PKG_SOURCE_SUBDIR)
 CUR_BUILD_DIR:=$(HOST_BUILD_DIR)-$(VARIANT)
+PATCH_DIR:=$(PATH_PREFIX)/patches
 
 include $(INCLUDE_DIR)/toolchain-build.mk