Fix Header Guard generation

All symbols that start with one underscore followed by uppercase letter,
or all symbols that contain two underscores are reserved for the
compiler and the standard library. They should not be used in user code.

crc_symtable.py

@@ -195,14 +195,14 @@ def _pretty_header_filename(filename):
 
 def _pretty_hdrprotection(opt):
     """
-    Return the name of a C header protection (e.g. __CRC_IMPLEMENTATION_H__).
+    Return the name of a C header protection (e.g. CRC_IMPLEMENTATION_H).
     """
     if opt.output_file is None:
         filename = 'pycrc_stdout'
     else:
         filename = os.path.basename(opt.output_file)
     out_str = ''.join([s.upper() if s.isalnum() else '_' for s in filename])
-    return '__' + out_str + '__'
+    return out_str
 
 
 def _get_underlying_crc_t(opt):