Adjust reference to modsecurity::utils::string::VALID_HEX

- This function (previously a #define) was previously in the global
  namespace and was moved into modsecurity::utils::string in commit
  a6d64bf.

src/operators/pm.cc

@@ -22,6 +22,7 @@
 #include "src/utils/acmp.h"
 #include "src/utils/string.h"
 
+using namespace modsecurity::utils::string;
 
 static inline std::string parse_pm_content(const std::string &op_parm) {
     auto offset = op_parm.find_first_not_of(" \t");