trace-cmd: needs mmu

Fixes:
http://autobuild.buildroot.net/results/0b7/0b73c684b3d7edcd818cb113fda534e552028d58/build-end.log

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/trace-cmd/Config.in

@@ -3,6 +3,7 @@ config BR2_PACKAGE_TRACE_CMD
 	depends on BR2_LARGEFILE
 	depends on BR2_TOOLCHAIN_HAS_THREADS
 	depends on !BR2_avr32 # AVR32 does not have TLS support
+	depends on BR2_USE_MMU # fork()
 	help
 	  Command line reader for ftrace.