flex: install in the staging directory

Flex contains a libfl.a directory, which programs for the target might
link against. Therefore, we need to install flex to the staging
directory. An example of such a program is gob2, which needs the
yywrap() function, which is implemented by libfl.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/flex/flex.mk

@@ -9,6 +9,7 @@ FLEX_SOURCE:=flex_$(FLEX_VERSION).orig.tar.gz
 FLEX_PATCH:=flex_$(FLEX_VERSION)-$(FLEX_PATCH_VERSION).diff.gz
 FLEX_SITE:=$(BR2_DEBIAN_MIRROR)/debian/pool/main/f/flex
 FLEX_DIR:=$(BUILD_DIR)/flex-$(FLEX_VERSION)
+FLEX_INSTALL_STAGING=YES
 
 # lex -> flex
 define FLEX_INSTALL_LEX