Fix build errors for C

/usr/include/opencc/opencc.h:95:15: error: unknown type name 'size_t'
OPENCC_EXPORT size_t opencc_convert_utf8_to_buffer(opencc_t opencc,

/usr/include/opencc/opencc.h:97:52: error: unknown type name 'size_t'
size_t length,

/usr/include/opencc/opencc.h:117:41: error: unknown type name 'size_t'
size_t length);

/usr/include/opencc/opencc.h:147:1: error: unknown type name 'namespace'
namespace opencc {

/usr/include/opencc/opencc.h:147:18: error: expected '=', ',', ';', 'asm' or 'attribute' before '{' token
namespace opencc {