Android: generate symlinks for all enabled gallium drivers

Current post install command relies on GALLIUM_TARGET_DRIVERS variable,
however variable needs to be initialized in src/gallium/Android.mk
in order that all enabled gallium drivers symlinks are correctly generated.

At the moment due to sorting of INC_DIRS and variable set with svga (vmwgfx)
only vmwgfx_dri.so and virtio_gpu_dri.so symlinks are generated.

Fixes: a3d98ca62f ("Android: use symlinks for driver loading")
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

src/gallium/Android.mk

@@ -25,6 +25,7 @@
 
 GALLIUM_TOP := $(call my-dir)
 GALLIUM_COMMON_MK := $(GALLIUM_TOP)/Android.common.mk
+GALLIUM_TARGET_DRIVERS :=
 
 SUBDIRS := auxiliary
 SUBDIRS += auxiliary/pipe-loader

src/gallium/drivers/svga/Android.mk

@@ -40,6 +40,6 @@ include $(GALLIUM_COMMON_MK)
 include $(BUILD_STATIC_LIBRARY)
 
 ifneq ($(HAVE_GALLIUM_VMWGFX),)
-GALLIUM_TARGET_DRIVERS := vmwgfx
+GALLIUM_TARGET_DRIVERS += vmwgfx
 $(eval GALLIUM_LIBS += $(LOCAL_MODULE) libmesa_winsys_svga)
 endif