net: fix implicit kmod.h usage in bridge/br_stp_if.c

To fix this, once the implicit presence of module.h is removed:

net/bridge/br_stp_if.c: In function ‘br_stp_start’:
net/bridge/br_stp_if.c:131: error: implicit declaration of function ‘call_usermodehelper’
net/bridge/br_stp_if.c:131: error: ‘UMH_WAIT_PROC’ undeclared (first use in this function)
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

net/bridge/br_stp_if.c

@@ -12,6 +12,7 @@
  */
 
 #include <linux/kernel.h>
+#include <linux/kmod.h>
 #include <linux/etherdevice.h>
 #include <linux/rtnetlink.h>