Fix android build

Fix android build since commit

696148e2 "Fix Event implementation"

by @ShadowNinja.

Fixes #3444. Thanks @kwolekr for pointing out the fix.

build/android/jni/Android.mk

@@ -349,6 +349,7 @@ LOCAL_SRC_FILES += deps/sqlite/sqlite3.c
 
 # Threading
 LOCAL_SRC_FILES += \
+		jni/src/threading/event.cpp \
 		jni/src/threading/mutex.cpp \
 		jni/src/threading/semaphore.cpp \
 		jni/src/threading/thread.cpp