package/libss7: add -fPIC

Fixes:
 - No autobuilder failures

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit e3f169fa78514691fa85dbdf5e6a51ff8301241c)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libss7/libss7.mk

@@ -36,7 +36,8 @@ endif
 LIBSS7_UTILS = parser_debug ss7test ss7linktest
 
 define LIBSS7_BUILD_CMDS
-	$(TARGET_MAKE_ENV) $(MAKE) $(TARGET_CONFIGURE_OPTS) -C $(@D) \
+	$(TARGET_MAKE_ENV) $(MAKE) $(TARGET_CONFIGURE_OPTS) \
+		CFLAGS="$(TARGET_CFLAGS) -fPIC" -C $(@D) \
 		$(LIBSS7_LIBS) $(LIBSS7_UTILS)
 endef