aco/isel: Compile all helper functions with static linkage

Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6504>

src/amd/compiler/aco_instruction_selection.cpp

@@ -10450,8 +10450,6 @@ Pseudo_instruction *add_startpgm(struct isel_context *ctx)
    return instr;
 }
 
-} /* end namespace */
-
 void fix_ls_vgpr_init_bug(isel_context *ctx, Pseudo_instruction *startpgm)
 {
    assert(ctx->shader->info.stage == MESA_SHADER_VERTEX);
@@ -10833,6 +10831,8 @@ void ngg_emit_nogs_output(isel_context *ctx)
    }
 }
 
+} /* end namespace */
+
 void select_program(Program *program,
                     unsigned shader_count,
                     struct nir_shader *const *shaders,