[build] Allow sparse to find compiler.h

sparse seems to have problems finding compiler.h when specified as
"-include compiler.h"; one possible explanation is that it ignores the
include path.  Fix by using "-include include/compiler.h".

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

@@ -475,7 +475,7 @@ LDFLAGS		+= --gc-sections
 
 # compiler.h is needed for our linking and debugging system
 #
-CFLAGS		+= -include compiler.h
+CFLAGS		+= -include include/compiler.h
 
 # CFLAGS for specific object types
 #