ccache: uses fork(), therefore only usable on MMU platforms

Fixes:

  http://autobuild.buildroot.org/results/2ba0eb693d24ac4ed9979762db234bfff1952c69/build-end.log

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

package/ccache/Config.in

@@ -1,5 +1,7 @@
 config BR2_PACKAGE_CCACHE
 	bool "ccache"
+	# needs fork()
+	depends on BR2_USE_MMU
 	help
 	  ccache is a compiler cache. It speeds up recompilation by
 	  caching previous compilations and detecting when the same