QMake: fixup a mistake when porting to QRE

The original expression seems to have been accidentally
changed during the port.

Amends a1947aeffe158a0ea7de3ced1bf8d6a4719a27ef.

Change-Id: I87821e1e025621a5efaf7a1e4f946fd3109fb256
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

qmake/generators/makefile.cpp

@@ -1678,7 +1678,7 @@ MakefileGenerator::replaceExtraCompilerVariables(
                 fullVal = val.join(' ');
             }
             ret.replace(match.capturedStart(), match.capturedLength(), fullVal);
-            rep = match.capturedStart(), fullVal.length();
+            rep += fullVal.length();
         } else {
             rep = match.capturedEnd();
         }