V4L/DVB (5745): Dvb: use '+=' instead of '=' for EXTRA_CFLAGS

The Makefiles in the dvb directory tree used '=' when defining EXTRA_CFLAGS
rather than '+=', which is far more common in the rest of the kernel source.
  =  14 times (9 of which this patch removes)
 :=  25 times
 += 123 times
This change also has certain advantages for the out of kernel v4l-dvb build
system.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>
diff --git a/drivers/media/dvb/b2c2/Makefile b/drivers/media/dvb/b2c2/Makefile
index bff00b5..e97ff60 100644
--- a/drivers/media/dvb/b2c2/Makefile
+++ b/drivers/media/dvb/b2c2/Makefile
@@ -12,4 +12,4 @@
 b2c2-flexcop-usb-objs = flexcop-usb.o
 obj-$(CONFIG_DVB_B2C2_FLEXCOP_USB) += b2c2-flexcop-usb.o
 
-EXTRA_CFLAGS = -Idrivers/media/dvb/dvb-core/ -Idrivers/media/dvb/frontends/
+EXTRA_CFLAGS += -Idrivers/media/dvb/dvb-core/ -Idrivers/media/dvb/frontends/