[gdb/testsuite] Add kfail in gdb.dwarf2/dwzbuildid.exp

[gdb/testsuite] Add kfail in gdb.dwarf2/dwzbuildid.exp

When running test-case gdb.dwarf2/dwzbuildid.exp using target board readnow, I
get:
...
(gdb) file dwzbuildid-mismatch^M
Reading symbols from dwzbuildid-mismatch...^M
warning: File "dwzbuildid5.o" has a different build-id, file skipped^M
could not find '.gnu_debugaltlink' file for dwzbuildid-mismatch^M
(gdb) delete breakpoints^M
(gdb) info breakpoints^M
No breakpoints or watchpoints.^M
(gdb) break -qualified main^M
No symbol table is loaded.  Use the "file" command.^M
Make breakpoint pending on future shared library load? (y or [n]) n^M
(gdb) FAIL: gdb.dwarf2/dwzbuildid.exp: mismatch: gdb_breakpoint: set breakpoint at main
...

This is PR symtab/26797: when using readnow, a failure in reading the dwarf
results in the minimal symbols not being available.

Add a corresponding KFAIL.

Tested on x86_64-linux.