doxygen input needs HIP_PATH environment variable

Change-Id: I05e027b765fde6339693564bb9dcc584a4ce43bd

docs/Makefile

@@ -2,4 +2,4 @@ HIP_PATH ?= ..
 DOXYGEN_SRC=$(wildcard $(HIP_PATH)/include/hcc_detail/*.h) $(wildcard $(HIP_PATH)/include/*.h) $(HIP_PATH)/src/hip_hcc.cpp $(wildcard doxygen-input/*)
 
 doc:  $(DOXYGEN_SRC)
-	doxygen doxygen-input/doxy.cfg
+	HIP_PATH=$(HIP_PATH) doxygen doxygen-input/doxy.cfg