avcodec/aac_defines: Add missing () to AAC_HALF_SUM() macro

Fixes: runtime error: shift exponent 1073741848 is too large for 32-bit type 'INTFLOAT' (aka 'int')
Fixes: 1880/clusterfuzz-testcase-minimized-4900645322620928

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
