btrfs-progs: Fix build with external toolchains

TARGET_CPPFLAGS were not propagated correctly leading to these kinds of
build failure:

btrfstune.c:26:23: fatal error: uuid/uuid.h: No such file or directory
 #include <uuid/uuid.h>
                       ^
compilation terminated.
btrfs-find-root.c:24:18: fatal error: zlib.h: No such file or directory
 #include <zlib.h>
                  ^
compilation terminated.
mkfs/common.c:18:23: fatal error: uuid/uuid.h: No such file or directory
 #include <uuid/uuid.h>

Fix that by passing TARGET_CPPFLAGS via EXTRA_CFLAGS

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

utils/btrfs-progs/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=btrfs-progs
 PKG_VERSION:=4.11
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PKG_SOURCE:=$(PKG_NAME)-v$(PKG_VERSION).tar.xz
 PKG_SOURCE_URL:=@KERNEL/linux/kernel/people/kdave/btrfs-progs/
@@ -53,6 +53,8 @@ CONFIGURE_ARGS += \
 	--disable-convert \
 	--disable-documentation
 
+EXTRA_CFLAGS=$(TARGET_CPPFLAGS)
+
 define Package/btrfs-progs/install
 	$(INSTALL_DIR) $(1)/usr/lib
 	$(INSTALL_BIN) $(PKG_INSTALL_DIR)/usr/lib/libbtrfs.so* $(1)/usr/lib