Fix cython definition to match c definition

This is required to compile with Cython options that
are more strict about const correctness.

src/python/grpcio/grpc/_cython/_cygrpc/grpc.pxi

@@ -523,7 +523,7 @@ cdef extern from "grpc/compression.h":
   int grpc_compression_algorithm_parse(
       grpc_slice value, grpc_compression_algorithm *algorithm) nogil
   int grpc_compression_algorithm_name(grpc_compression_algorithm algorithm,
-                                      char **name) nogil
+                                      const char **name) nogil
   grpc_compression_algorithm grpc_compression_algorithm_for_level(
       grpc_compression_level level, uint32_t accepted_encodings) nogil
   void grpc_compression_options_init(grpc_compression_options *opts) nogil

src/python/grpcio/grpc/_cython/_cygrpc/records.pyx.pxi

@@ -782,7 +782,7 @@ cdef class CompressionOptions:
 
 
 def compression_algorithm_name(grpc_compression_algorithm algorithm):
-  cdef char* name
+  cdef const char* name
   with nogil:
     grpc_compression_algorithm_name(algorithm, &name)
   # Let Cython do the right thing with string casting