libpjsip: fix m68k uclinux compile

The GCC manual suggest when getting:
relocation truncated to fit: R_68K_GOT16O foobar
to use -mxgot.

Fixes:

  http://autobuild.buildroot.org/results/e8cdfaf8e2da29a855b5bc09774f3aadee087737/

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/libpjsip/libpjsip.mk

@@ -13,9 +13,16 @@ LIBPJSIP_LICENSE_FILES = COPYING
 LIBPJSIP_INSTALL_STAGING = YES
 LIBPJSIP_MAKE = $(MAKE1)
 
+LIBPJSIP_CFLAGS = $(TARGET_CFLAGS) -DPJ_HAS_IPV6=1
+
+# relocation truncated to fit: R_68K_GOT16O
+ifeq ($(BR2_m68k_cf),y)
+LIBPJSIP_CFLAGS += -mxgot
+endif
+
 LIBPJSIP_CONF_ENV = \
 	LD="$(TARGET_CC)" \
-	CFLAGS="$(TARGET_CFLAGS) -DPJ_HAS_IPV6=1"
+	CFLAGS="$(LIBPJSIP_CFLAGS)"
 
 LIBPJSIP_CONF_OPTS = \
 	--disable-sound \