nir/builder: fix c++11 compiler warning

nir/builder: fix c++11 compiler warning

Fixes:

   In file included from nir/nir_lower_samplers.cpp:27:0:
   nir/nir_builder.h: In function 'nir_ssa_def* nir_channel(nir_builder*, nir_ssa_def*, int)':
   nir/nir_builder.h:222:37: warning: narrowing conversion of 'c' from 'int' to 'unsigned int' inside { } is ill-formed in C++11 [-Wnarrowing]
       unsigned swizzle[4] = {c, c, c, c};

Signed-off-by: Rob Clark <robclark@freedesktop.org>