gmp: requires host-m4

On a system where m4 is not installed, gmp does not build due to the
lack of m4. Add the missing host-m4 dependency.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/gmp/gmp.mk

@@ -11,6 +11,8 @@ GMP_INSTALL_STAGING = YES
 GMP_LICENSE = LGPLv3+
 GMP_LICENSE_FILES = COPYING.LIB
 
+GMP_DEPENDENCIES = host-m4
+
 # Bad ARM assembly breaks on pure thumb
 ifeq ($(ARCH),arm)
 GMP_MAKE_OPT += CFLAGS="$(TARGET_CFLAGS) -marm"