package/mutt: fix build failure when host provides an old gpgrt-config

As already done for gnupg2 in commit
d7f2d8403ec82f3b1772ec17bf1df8c42987a1f8, apply the same fix to mutt to
avoid the following build failure:

/home/thomas/autobuild/instance-2/output-1/host/bin/i686-buildroot-linux-uclibc-gcc -DPKGDATADIR=\"/usr/share/mutt\" -DSYSCONFDIR=\"/etc\" -DBINDIR=\"/usr/bin\" -DMUTTLOCALEDIR=\"/usr/share/locale\" -DHAVE_CONFIG_H=1 -I.  -I. -I.   -I/usr/include -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Wall -pedantic -Wno-long-long -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -O3 -g0  -c -o txt2c.o txt2c.c
i686-buildroot-linux-uclibc-gcc: ERROR: unsafe header/library path used in cross-compilation: '-I/usr/include'

Fixes:
 - http://autobuild.buildroot.org/results/98010be4ef70e58819ea2d17315bdac66a619b23

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit a39e328bb2835cd157ee1622d5117ba252d44733)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/mutt/mutt.mk

@@ -37,6 +37,10 @@ MUTT_DEPENDENCIES += libgpgme
 MUTT_CONF_OPTS += \
 	--enable-gpgme \
 	--with-gpgme-prefix=$(STAGING_DIR)/usr
+
+# Force the path to "gpgrt-config" (from the libgpg-error package) to
+# avoid using the one on host, if present.
+MUTT_CONF_ENV += GPGRT_CONFIG=$(STAGING_DIR)/usr/bin/gpgrt-config
 else
 MUTT_CONF_OPTS += --disable-gpgme
 endif