gcc: hide shared libgcc option if BR2_PREFER_STATIC_LIB is enabled

Shared libgcc without shared libs doesn't make much sense.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

toolchain/gcc/Config.in

@@ -153,6 +153,7 @@ config BR2_INSTALL_FORTRAN
 
 config BR2_GCC_SHARED_LIBGCC
 	bool "Build/install a shared libgcc?"
+	depends on !BR2_PREFER_STATIC_LIB
 	default y
 	depends on !BR2_sparc && !BR2_sparc64
 	help