emulator opengl: fixing clean windows build

Fixes windows build found after clean build.
the emugen tool does not built on windows. For
windows build we need to use the build host executable.
(that works for mingw builds since it uses the linux exeutable).

Change-Id: I6a0cc6936d4b9f11f074695066b800e12156fd83
diff --git a/tools/emulator/opengl/host/tools/emugen/Android.mk b/tools/emulator/opengl/host/tools/emugen/Android.mk
index 51330ec..ad9ab06 100644
--- a/tools/emulator/opengl/host/tools/emugen/Android.mk
+++ b/tools/emulator/opengl/host/tools/emugen/Android.mk
@@ -17,4 +17,10 @@
 # protocol encoders/ decoders. This variable is used by other emugl modules.
 EMUGL_EMUGEN := $(LOCAL_BUILT_MODULE)
 
+else # windows build
+
+# on windows use the build host emugen executable
+# (that will be the linux exeutable when using mingw build)
+EMUGL_EMUGEN := $(BUILD_OUT_EXECUTABLES)/emugen
+
 endif