fix Makefile build include file dependency tracking

Minor mistake that meant that make builds were completely ignoring
includes as dependencies, which is no good during development.

(Clean builds were not affected since for a clean build the include
deps are irrelevant)

Author: johnbartholomew

Makefile

@@ -165,7 +165,7 @@ PLAIN_TEST_BINS := $(basename $(notdir $(PLAIN_TEST_SRC)))
 GTEST_TEST_BINS := $(basename $(notdir $(GTEST_TEST_SRC)))
 ALL_TEST_BINS := $(PLAIN_TEST_BINS) $(GTEST_TEST_BINS)
 
-DEPS_FILES := $(addprefix .makebuild/,$(addsuffix .d,$(LIB_SRC) $(LIB_CPP_SRC) $(BINS_SRC) $(TEST_SRC)))
+DEPS_FILES := $(addprefix .makebuild/,$(addsuffix .o.d,$(LIB_SRC) $(LIB_CPP_SRC) $(BINS_SRC) $(TEST_SRC)))
 # Intermediate build output directories.
 BUILD_DIRS := $(sort $(dir $(DEPS_FILES)) .makebuild/stdlib/)