Added missing .c_str() to convert the string object
into a character string.

Change-Id: I3810f67a2462f7717fdec61679e09a191b043adf
Reviewed-on: https://swiftshader-review.googlesource.com/4878Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>