d3d12: Fix GCC warnings for missing function prototypes

Most of these are adding 'static', for functions that are local
to a translation unit but weren't declared static.

There's one instance of a missing include for bringing the prototype
into the translation unit, one function missing a return type (default-int),
and one which added inline to avoid it being considered unused in some sources.

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7780>

src/microsoft/compiler/dxil_nir.c

@@ -417,7 +417,7 @@ lower_store_ssbo(nir_builder *b, nir_intrinsic_instr *intr)
    return true;
 }
 
-void
+static void
 lower_load_vec32(nir_builder *b, nir_ssa_def *index, unsigned num_comps, nir_ssa_def **comps, nir_intrinsic_op op)
 {
    for (unsigned i = 0; i < num_comps; i++) {
@@ -719,7 +719,7 @@ dxil_nir_lower_ubo_to_temp(nir_shader *nir)
    return progress;
 }
 
-bool
+static bool
 lower_load_ubo(nir_builder *b, nir_intrinsic_instr *intr)
 {
    assert(intr->dest.is_ssa);
@@ -788,6 +788,7 @@ dxil_nir_lower_loads_stores_to_dxil(nir_shader *nir)
    return progress;
 }
 
+static bool
 lower_shared_atomic(nir_builder *b, nir_intrinsic_instr *intr,
                     nir_intrinsic_op dxil_op)
 {