#183 fix build with gcc 10

[Fabrice Fontaine]

Define setjmp_buffer as static to avoid the following build failure with
gcc 10 (which defaults to -fno-common):

/home/buildroot/autobuild/instance-1/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/10.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: image_utils.o:(.bss+0x0): multiple definition of `setjmp_buffer'; metadata.o:(.bss+0x0): first defined here
collect2: error: ld returned 1 exit status

Fixes:
- http://autobuild.buildroot.org/results/8754bb4f7d749f999d5f8ddfec587470ceec4476

[Sérgio M. Basto]

this commit was applied here https://sourceforge.net/p/minidlna/git/ci/b5e75ff7d160a02632cab416ff0af66504c7db8b please close as fixed