FIXED_PHY must depend on PHYLIB=y

This patch fixes the following build error introduced by commit
a79d8e9 and reported by Olaf Hering:

<--  snip  -->

...
  LD      .tmp_vmlinux1
arch/powerpc/sysdev/built-in.o: In function `of_add_fixed_phys':
fsl_soc.c:(.init.text+0xd34): undefined reference to `fixed_phy_add'
make: *** [.tmp_vmlinux1] Error 1

<--   snip  -->

Signed-off-by: Adrian Bunk <adrian.bunk@movial.fi>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/net/phy/Kconfig

@@ -67,6 +67,7 @@ config REALTEK_PHY
 
 config FIXED_PHY
 	bool "Driver for MDIO Bus/PHY emulation with fixed speed/link PHYs"
+	depends on PHYLIB=y
 	---help---
 	  Adds the platform "fixed" MDIO Bus to cover the boards that use
 	  PHYs that are not connected to the real MDIO bus.