Changed declarations to better C++ init

Author: eugeneepshteyn

flang-rt/lib/runtime/extensions.cpp

@@ -288,8 +288,8 @@ int FORTRAN_PROCEDURE_NAME(hostnm)(char *hn, int length) {
   if (status == 0) {
     // Find zero terminator and fill the string from the
     // zero terminator to the end with spaces
-    char *str_end = hn + length;
-    char *str_zero = std::find(hn, str_end, '\0');
+    char *str_end{hn + length};
+    char *str_zero{std::find(hn, str_end, '\0')};
     std::fill(str_zero, str_end, ' ');
   }