FIXED_PHY must depend on PHYLIB=y

This patch fixes the following build error introduced by commit
a79d8e93d300adb84cccc38ac396cfb118c238ad 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>
diff --git a/drivers/net/phy/Kconfig b/drivers/net/phy/Kconfig
index f4ca059..3ac8529 100644
--- a/drivers/net/phy/Kconfig
+++ b/drivers/net/phy/Kconfig
@@ -67,6 +67,7 @@
 
 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.