mainboard/google/reef: declare mainboard_ec_init() in each C file

There's no common EC header file in the code base, and I didn't
want to use a header file for single declaration. Therefore,
just move the declaration to each file that uses that symbol.

BUG=chrome-os-partner:56677

Change-Id: Ibaebb0ea6a07029aec02d5185cf05ffb8593b117
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/16420
Tested-by: build bot (Jenkins)
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>

src/mainboard/google/reef/mainboard.c

@@ -25,6 +25,8 @@
 #include <variant/ec.h>
 #include <variant/gpio.h>
 
+void mainboard_ec_init(void);
+
 static void mainboard_init(void *chip_info)
 {
 	int boardid;