phytool: add kernel headers dependency

phytool includes linux/mdio.h, that is available since
kernel 3.7. So add related dependency.

Fixes:
http://autobuild.buildroot.net/results/e0e/e0eebcb9a694cd5d4c76c40e5dba971e3ac9d7c5/

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

package/phytool/Config.in

@@ -1,6 +1,10 @@
 config BR2_PACKAGE_PHYTOOL
 	bool "phytool"
+	depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_7 # linux/mdio.h
 	help
 	  Linux MDIO register access utility.
 
 	  https://github.com/wkz/phytool
+
+comment "phytool needs a toolchain w/ headers >= 3.7"
+	depends on !BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_7