staging:rtl8192u: Fix smatch warning of function definition with external linkage

This patch fixes the following smatch warning in r8192U_core.c -
drivers/staging/rtl8192u/r8192U_core.c:3539:13: warning: function 'rtl819x_watchdog_wqcallback' with external linkage has definition

Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>