microsoft/compiler: Add dxil_nir_lower_16bit_conv prototype.

Fix missing-prototypes build error.

src/microsoft/compiler/dxil_nir_algebraic.c:5761:1: error: no previous prototype for ‘dxil_nir_lower_16bit_conv’ [-Werror=missing-prototypes] 5761 | dxil_nir_lower_16bit_conv(nir_shader *shader) | ^~~~~~~~~~~~~~~~~~~~~~~~~

Fixes: b9c61379 ("microsoft/compiler: translate nir to dxil") Signed-off-by: Vinson Lee vlee@freedesktop.org

cc: @kusma