Add -lz to the linker flags to fix linking error.

Not sure if i'm doing something wrong, but linking process fails for me with the following error:

```Linking CXX executable kcov
/usr/local/lib64/libdw.a(dwarf_begin_elf.o): In function `check_section.isra.1':
(.text+0x1de): undefined reference to `inflateInit_'
/usr/local/lib64/libdw.a(dwarf_begin_elf.o): In function `check_section.isra.1':
(.text+0x217): undefined reference to `inflate'
/usr/local/lib64/libdw.a(dwarf_begin_elf.o): In function `check_section.isra.1':
(.text+0x22a): undefined reference to `inflateReset'
/usr/local/lib64/libdw.a(dwarf_begin_elf.o): In function `check_section.isra.1':
(.text+0x244): undefined reference to `inflateEnd'```

This patch fixes the issue for me.