extract_elf_ramdisk: Increase buffer for uncompressed ramdisks

This fixes the following error (when the uncompressed ramdisk is bigger than 8mb):

gunzip error -- Err:inflate -5
Failed to gunzip

Use a bigger buffer (13 MB) to make sure this won't happen again.

Needed especially for TWRP(-based) recovery ramdisks (which are sometimes about ~12 MB big)

Change-Id: I59db6a8fff47876d4b4b535b24295484a98ab2ba