travis: install llvm-* to get llvm-symbolizer

travis: install llvm-* to get llvm-symbolizer

Backtraces like
```
misc-utils/cal.c:295:4: runtime error: signed integer overflow: 2147483647 + 5 cannot be represented in type 'int'
==1405==WARNING: invalid path to external symbolizer!
==1405==WARNING: Failed to use and restart external symbolizer!
    #0 0x4c4c09  (/home/travis/build/karelzak/util-linux/cal+0x4c4c09)
    #1 0x7f4363046b96  (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
    #2 0x41bab9  (/home/travis/build/karelzak/util-linux/cal+0x41bab9)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior misc-utils/cal.c:295:4 in
```
aren't very helpful

It's follow-up to https://github.com/karelzak/util-linux/pull/1075