Add -Wno-unused-parameter to suppress warnings.

For example:
external/srtp/crypto/cipher/null_cipher.c:110:48: error: unused
parameter 'iv' [-Werror=unused-parameter]

Change-Id: Ieaacecd557f03bf4fe2a841aa9ef43a6b2a84c31
diff --git a/Android.mk b/Android.mk
index 6e72b23..569c802 100644
--- a/Android.mk
+++ b/Android.mk
@@ -33,7 +33,8 @@
     -Werror \
     -Wno-ignored-qualifiers \
     -Wno-sign-compare \
-    -Wno-missing-field-initializers
+    -Wno-missing-field-initializers \
+    -Wno-unused-parameter
 
 common_C_INCLUDES = $(LOCAL_PATH)/include