libmedia: Fix build with gcc 4.7

gcc 4.7 doesn't like the redefinition of n in the same
scope:
for(size_t n=...;;) {
  size_t n = 2; // <-- duplicate definition
}

Change-Id: Idfc2e768a0eb85a3dacccd3431b4ad44775432a4
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>