broadcom-wl: define module directories

Define the module subdirs so our build system properly picks up the
Module.symvers.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>

package/kernel/broadcom-wl/Makefile

@@ -22,6 +22,8 @@ ifeq ($(ARCH),mips)
 PKG_HASH:=ca6a86ca3e3e9c85b6dbb665b35bcbf338c37829c1b2f1994487d55664886045
 endif
 
+PKG_EXTMOD_SUBDIRS:=driver driver-mini glue
+
 PKG_USE_MIPS16:=0
 PKG_FLAGS:=nonshared