Fix duplicated symbols
Fix duplicated symbols caused by having definition instead of declaration in
header file.
diff --git a/src/php/ext/grpc/call.h b/src/php/ext/grpc/call.h
index 827e9a2..bce5d82 100644
--- a/src/php/ext/grpc/call.h
+++ b/src/php/ext/grpc/call.h
@@ -57,7 +57,7 @@
} while (0)
/* Class entry for the Call PHP class */
-zend_class_entry *grpc_ce_call;
+extern zend_class_entry *grpc_ce_call;
/* Wrapper struct for grpc_call that can be associated with a PHP object */
typedef struct wrapped_grpc_call {