[build] Use -Wno-decl when running sparse

Linker table entries must be non-static in order to avoid being
completely optimised away by some versions of gcc.  Use -Wno-decl to
prevent sparse from warning about these, since the alternative would
be to litter the code with otherwise unnecessary "extern"
declarations.

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

src/Makefile.housekeeping

 ifeq ($(C),1)
 export REAL_CC := $(CC)
 CC := cgcc
+CFLAGS += -Wno-decl
 endif
 
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