r300: Print warning when stubbing derivatives

Fixes: e1c640c3a4
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14669>

src/gallium/drivers/r300/compiler/radeon_program_alu.c

@@ -38,6 +38,7 @@
 #include "radeon_compiler.h"
 #include "radeon_compiler_util.h"
 
+#include "util/log.h"
 
 static struct rc_instruction *emit1(
 	struct radeon_compiler * c, struct rc_instruction * after,
@@ -1100,6 +1101,10 @@ int radeonStubDeriv(struct radeon_compiler* c,
 	inst->U.I.Opcode = RC_OPCODE_MOV;
 	inst->U.I.SrcReg[0].Swizzle = RC_SWIZZLE_0000;
 
+	mesa_logw_once("r300: WARNING: Shader is trying to use derivatives, "
+					"but the hardware doesn't support it. "
+					"Expect possible misrendering (it's not a bug, do not report it).");
+
 	return 1;
 }