libpisp: include <string> explicitly

The pisp_utils references std::string without directly pulling in
the <string> header.

Bring in the include explicitly to prevent compiler warnings.

Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libpisp/common/pisp_utils.cpp

@@ -8,6 +8,7 @@
 #include <cmath>
 #include <cstdint>
 #include <map>
+#include <string>
 
 #include "backend/pisp_be_config.h"