Makefile.target: link with execinfo for nyx

[Wenzel]

debug.c needs to link with execinfo libs

    In file included from /usr/include/fortify/stdlib.h:29,
                     from /build/kafl/qemu/include/qemu/osdep.h:98,
                     from /build/kafl/qemu/accel/kvm/kvm-all.c:18:
    /usr/include/limits.h:97: note: this is the location of the previous definition
       97 | #define PAGE_SIZE PAGESIZE
          |
    /build/kafl/qemu/hw/rtc/mc146818rtc.c:85:6: warning: no previous prototype for 'enable_fast_snapshot_rtc' [-Wmissing-prototypes]
       85 | void enable_fast_snapshot_rtc(void){
          |      ^~~~~~~~~~~~~~~~~~~~~~~~
    /build/kafl/qemu/hw/i386/kvm/clock.c:43:6: warning: no previous prototype for 'enable_fast_snapshot_kvm_clock' [-Wmissing-prototypes]
       43 | void enable_fast_snapshot_kvm_clock(void){
          |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /usr/lib/gcc/x86_64-alpine-linux-musl/11.2.1/../../../../x86_64-alpine-linux-musl/bin/ld: nyx/debug.o: in function `qemu_backtrace':
    /build/kafl/qemu/nyx/debug.c:20: undefined reference to `backtrace'
    /usr/lib/gcc/x86_64-alpine-linux-musl/11.2.1/../../../../x86_64-alpine-linux-musl/bin/ld: /build/kafl/qemu/nyx/debug.c:24: undefined reference to `backtrace_symbols'
    collect2: error: ld returned 1 exit status
    make[3]: *** [Makefile:209: qemu-system-x86_64] Error 1
    make[2]: *** [Makefile:491: x86_64-softmmu/all] Error 2