Solve warnings by adding headers not included by grpc.h

The two log.h can’t be listed, though, as they include the nefarious
<inttypes.h> (See discussing at BoringSSL.podspec). Not listing them
seems to be alright, though.

include/grpc/module.modulemap

 framework module grpc {
   umbrella header "grpc.h"
+
+  header "byte_buffer_reader.h"
+  header "grpc_security.h"
+  header "grpc_security_constants.h"
+  header "impl/codegen/alloc.h"
+  header "impl/codegen/byte_buffer_reader.h"
+  header "support/alloc.h"
+  header "support/port_platform.h"
+  header "support/string_util.h"
+
   export *
   module * { export * }
 }