tools: Make sure to create the tools directory before building decode_simple.o

This directory dependency is normally added implicitly by rules
in ffbuild/common.mak; for tools it's created by a rule for TOOLOBJS.
TOOLOBJS is populated implicitly from TOOLS, and decode_simple.o
doesn't end up there because it's an odd occurrance of a lone
object file in the tools subdirectory, not belonging to any other
tool.

Signed-off-by: Martin Storsjö <martin@martin.st>

tools/Makefile

@@ -20,6 +20,8 @@ tools/target_io_dem_fuzzer.o: tools/target_dem_fuzzer.c
 tools/venc_data_dump$(EXESUF): tools/decode_simple.o
 tools/scale_slice_test$(EXESUF): tools/decode_simple.o
 
+tools/decode_simple.o: | tools
+
 OUTDIRS += tools
 
 clean::