microperl: uses fork(), only available on MMU platforms

Fixes
http://autobuild.buildroot.net/results/0d5e272b594bc6cc84df291134e2b23d33b97d17/build-end.log.

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

package/microperl/Config.in

@@ -1,5 +1,7 @@
 config BR2_PACKAGE_MICROPERL
 	bool "microperl"
+	# needs fork()
+	depends on BR2_USE_MMU
 	help
 	  Perl without operating-specific functions such as readdir.