nir: Allow using nir_eval_const_opcode in C++ code

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/39184>

src/compiler/nir/nir_constant_expressions.h

@@ -31,9 +31,17 @@
 #include "nir_defines.h"
 #include "nir_opcodes.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 void nir_eval_const_opcode(nir_op op, nir_const_value *dest,
                            unsigned num_components, unsigned bit_size,
                            nir_const_value **src,
                            unsigned float_controls_execution_mode);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* NIR_CONSTANT_EXPRESSIONS_H */