libcore/.../rg_apache_harmony_xml_ExpatParser: in C++, the return type

of strchr(const char*) is 'const char*' instead of 'char *'.

C++ overloads string functions so that strchr(char*) returns 'char*' and
strchr(const char*) returns 'const char*'.  This patch fixes an "invalid
conversion from ‘const char*’ to ‘char*’" error when building with gcc-4.4.