distcc: only available on MMU platforms

Fixes
http://autobuild.buildroot.org/results/3c1fa37c5b687767b53868f083910ca3f7dbcac0/build-end.log.

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

package/distcc/Config.in

@@ -1,5 +1,7 @@
 config BR2_PACKAGE_DISTCC
 	bool "distcc"
+	# needs fork()
+	depends on BR2_USE_MMU
 	help
 	  Distributed compiler client and server.  Allows you to
 	  distribute compilation of C code across several machines