Set EXTRAVERSION to -gfiber0

For consistency with other Google Fiber devices, set EXTRAVERSION to
-gfiber0. The kernel will then be known as 4.7.0-gfiber0

Change-Id: I01aadb288036ce8fb9947eb00974a0b91f5beeeb
diff --git a/Makefile b/Makefile
index 06e1720..aa72da2 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 VERSION = 4
 PATCHLEVEL = 7
 SUBLEVEL = 0
-EXTRAVERSION =
+EXTRAVERSION = -gfiber0
 NAME = Psychotic Stoned Sheep
 
 # *DOCUMENTATION*