libffi: remove invalid variable 'includedir' in .pc file

Fixes the following error:

  Variable 'includedir' not defined in '/home/daniel/buildroot/output/host/usr/arm-unknown-linux-gnueabi/sysroot/usr/lib/pkgconfig/libffi.pc'

Also, use $(SED) instead of 'sed -i'.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/libffi/libffi.mk

@@ -13,7 +13,8 @@ LIBFFI_INSTALL_STAGING = YES
 # accordingly
 define LIBFFI_MOVE_STAGING_HEADERS
 	mv $(STAGING_DIR)/usr/lib/libffi-*/include/*.h $(STAGING_DIR)/usr/include/
-	sed -i '/^includedir.*/d' $(STAGING_DIR)/usr/lib/pkgconfig/libffi.pc
+	$(SED) '/^includedir.*/d' $(STAGING_DIR)/usr/lib/pkgconfig/libffi.pc
+	$(SED) '/^Cflags:.*/d' $(STAGING_DIR)/usr/lib/pkgconfig/libffi.pc
 	rm -rf $(TARGET_DIR)/usr/lib/libffi-*
 endef
 
@@ -23,7 +24,8 @@ LIBFFI_POST_INSTALL_STAGING_HOOKS += LIBFFI_MOVE_STAGING_HEADERS
 define LIBFFI_MOVE_TARGET_HEADERS
 	install -d $(TARGET_DIR)/usr/include/
 	mv $(TARGET_DIR)/usr/lib/libffi-*/include/*.h $(TARGET_DIR)/usr/include/
-	sed -i '/^includedir.*/d' $(TARGET_DIR)/usr/lib/pkgconfig/libffi.pc
+	$(SED) '/^includedir.*/d' $(STAGING_DIR)/usr/lib/pkgconfig/libffi.pc
+	$(SED) '/^Cflags:.*/d' $(STAGING_DIR)/usr/lib/pkgconfig/libffi.pc
 	rm -rf $(TARGET_DIR)/usr/lib/libffi-*
 endef