Fix (-W)documentation error found by Clang

Fix (-W)documentation error found by Clang

./str_util.h:72:11: error: parameter 'str' not found in the function declaration [-Werror,-Wdocumentation]
 * @param str The string to be checked
          ^~~
./str_util.h:72:11: note: did you mean 'string'?
 * @param str The string to be checked
          ^~~
          string
./str_util.h:80:11: error: parameter 'str' not found in the function declaration [-Werror,-Wdocumentation]
 * @param str The string to be checked
          ^~~
./str_util.h:80:11: note: did you mean 'string'?
 * @param str The string to be checked
          ^~~
          string

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@51252 f5534014-38df-0310-8fa8-9805f1628bb7