libelf/libdwfl: Remove LIB_SYSTEM_H define in lib{elf,dwfl}_crc32.c

libelf/libdwfl: Remove LIB_SYSTEM_H define in lib{elf,dwfl}_crc32.c

rationale: https://sourceware.org/bugzilla/show_bug.cgi?id=21001

If we don't remove this macro, when try #include <system.h> in
libdw/memory-access.h wont' take effect because
"#define LIB_SYSTEM_H	1"
The compile error:
./../libdw/memory-access.h:390:12: error: implicit declaration of
function ‘bswap_32’ [-Werror=implicit-function-declaration]

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>