toolchain: skip gcc/minimal for musl

No extra libc header build step is done, so no extra toolchain is needed
for preparing it.
This saves a significant amount of build time and disk space

Signed-off-by: Felix Fietkau <nbd@nbd.name>

toolchain/uClibc/headers/Makefile

@@ -20,6 +20,7 @@ define Host/Install
 		ARCH="$(CONFIG_ARCH)" \
 		pregen \
 		install_headers
+	$(CP) $(BUILD_DIR_TOOLCHAIN)/linux-dev/* $(BUILD_DIR_TOOLCHAIN)/$(LIBC)-dev/
 endef
 
 $(eval $(call HostBuild))