libstagefright/matroska: Fix build with gcc 4.7

gcc 4.7 doesn't like the redefinition of i in the
same scope

Change-Id: Idf1b33af7041f0302f2f2562442a3e11ce9607d9
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>

media/libstagefright/matroska/MatroskaExtractor.cpp

@@ -768,12 +768,12 @@ void MatroskaExtractor::findThumbnails() {
         }
 
         BlockIterator iter(this, info->mTrackNum);
-        int32_t i = 0;
+        int32_t j = 0;
         int64_t thumbnailTimeUs = 0;
         size_t maxBlockSize = 0;
-        while (!iter.eos() && i < 20) {
+        while (!iter.eos() && j < 20) {
             if (iter.block()->IsKey()) {
-                ++i;
+                ++j;
 
                 size_t blockSize = 0;
                 for (int i = 0; i < iter.block()->GetFrameCount(); ++i) {