Suppress existing warnings in external/libgsm

Suppress existing warning types and enable -Werror to prevent
any new errors from being added.

I have filed a bug to fix these warnings a year ago, but I got
no traction, so suppressing these warnings for now.

Bug: 27152357
Test: None
Change-Id: I2e0f973769cb50e460031fdb661efea582ceaa74
(cherry picked from commit ea0db058810275c21fbf51f8487e0a1f8438a56b)
diff --git a/Android.mk b/Android.mk
index 8ec0b42..5d47d37 100644
--- a/Android.mk
+++ b/Android.mk
@@ -37,6 +37,8 @@
     -DSASR \
     -DWAV49 \
     -Wno-unused-parameter \
+    -Wno-shift-negative-value \
+    -Werror
 
 LOCAL_C_INCLUDES += $(LOCAL_PATH)/inc