Revert "dsputil: make {add/put/put_signed}_pixels_clamped() non-static."

This reverts commit 484a337c.

These functions were used in f8bed30d "VC1: merge idct8x8, coeff
adjustments and put_pixels" which was reverted in 18b6a69c.

Signed-off-by: Mans Rullgard <mans@mansr.com>