nir: rename nir_lower_samplers.c{pp,}

nir: rename nir_lower_samplers.c{pp,}

With the only C++ function having its own wrapper we can 'demote' this
file to a normal C one. This allows us to get rid of extern C { #include
<foo.h> } 'hacks'. Plus some of the headers may use C99 initializers,
which are not supported by the ISO standard.

This may cause build issue on incremental builds. If so run the
following:

sed -i -e 's|samplers\.cpp|samplers.c|' src/glsl/nir/.deps/nir_lower_samplers.Plo

Fixes: ef8eebc6ad5(nir: support indirect indexing samplers in struct arrays)
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reported-by: Gottfried Haider <gottfried.haider@gmail.com>
Tested-by: Gottfried Haider <gottfried.haider@gmail.com>
Reviewed-by: Timothy Arceri <t_arceri@yahoo.com.au>