media: Makefile: fix test drivers compilation

test_drivers/ folder is not being added by media Makefile, so it is not
being compiled.

Add test_drivers/ folder in Makefile folder's list.

Fixes: 4b32216a ("media: split test drivers from platform directory")
Signed-off-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/Makefile

@@ -29,6 +29,6 @@ obj-$(CONFIG_CEC_CORE) += cec/
 # Finally, merge the drivers that require the core
 #
 
-obj-y += common/ platform/ pci/ usb/ mmc/ firewire/ spi/
+obj-y += common/ platform/ pci/ usb/ mmc/ firewire/ spi/ test_drivers/
 obj-$(CONFIG_VIDEO_DEV) += radio/