package/mdio-tools: needs fork

Fix the following build failure raised since the addition of the package
in commit 8fdf8731e727dc5c2d4080d7a56e1d71dc25e533:

mdio.c: In function 'mdio_modprobe':
mdio.c:738:15: error: implicit declaration of function 'fork' [-Werror=implicit-function-declaration]
  738 |         pid = fork();
      |               ^~~~

Fixes:
 - http://autobuild.buildroot.org/results/c53aaeaa34dd4d6d9a57da196687beecaeed9fe2

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Joachim Wiberg <troglobit@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/mdio-tools/Config.in

@@ -1,12 +1,15 @@
 comment "mdio-tools needs a Linux kernel to be built"
+	depends on BR2_USE_MMU
 	depends on !BR2_LINUX_KERNEL
 
 comment "mdio-tools needs a toolchain w/ headers >= 5.1"
+	depends on BR2_USE_MMU
 	depends on BR2_LINUX_KERNEL
 	depends on !BR2_TOOLCHAIN_HEADERS_AT_LEAST_5_1
 
 config BR2_PACKAGE_MDIO_TOOLS
 	bool "mdio-tools"
+	depends on BR2_USE_MMU # fork()
 	depends on BR2_LINUX_KERNEL
 	depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_5_1
 	select BR2_PACKAGE_LIBMNL