include string header for colortemp.h

To fix mac clang compilation ````error: implicit instantiation of undefined template 'std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >'
    std::string method;````

rtengine/colortemp.h

@@ -21,6 +21,7 @@
 
 #include <cmath>
 #include <map>
+#include <string>
 
 namespace rtengine
 {