package/kexec-tools: fix build with gcc 10

package/kexec-tools: fix build with gcc 10

Build is broken with gcc 10 since bump to version 2.0.22 in commit
cadb8f2f317bf37c13aea98ac1c81bf8566aad92:

/tmp/instance-1/output-1/host/lib/gcc/powerpc-buildroot-linux-uclibc/10.3.0/../../../../powerpc-buildroot-linux-uclibc/bin/ld: kexec/arch/ppc/kexec-elf-ppc.o:(.sbss+0x0): multiple definition of `ramdisk'; kexec/arch/ppc/kexec-ppc.o:(.sbss+0x0): first defined here

Fixes:
 - http://autobuild.buildroot.org/results/22932529f925630ec6db3c6a4eaebbda68b3fc16

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>