dsp.h: fix -Wundef w/__mips_dsp_rev
dsp.h: fix -Wundef w/__mips_dsp_rev

Change-Id: I552a543c7b039774041b43ace75b0cbea566b119

PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp