fix(hsolver): add missing <string> include in simple_pexsi.h (#7933)

simple_pexsi.h uses std::string in the simplePEXSI declaration but
relies on transitive includes. Intel Classic (icpc) fails with
"namespace std has no member string"; include <string> explicitly.

Author: Flying-dragon-boxing

source/source_hsolver/module_pexsi/simple_pexsi.h

@@ -2,6 +2,7 @@
 #define SIMPLE_PEXSI_H
 
 #include <mpi.h>
+#include <string>
 // a simple interface for calling pexsi with 2D block cyclic distributed matrix
 namespace pexsi
 {