Merge pull request #4684 from BrettSchiff/include_string

add #include <string> to source_lines.h to fix build errors on VS2019

Author: tautschnig

src/goto-instrument/source_lines.h

@@ -22,6 +22,7 @@ Author: Mark R. Tuttle
 
 #include <map>
 #include <set>
+#include <string>
 
 class source_locationt;