[gdb/testsuite] Compile gdb.dwarf2/dw2-error.exp quietly

[gdb/testsuite] Compile gdb.dwarf2/dw2-error.exp quietly

When running test-case gdb.dwarf2/dw2-error.exp with target board
cc-with-gdb-index, we get:
...
Running src/gdb/testsuite/gdb.dwarf2/dw2-error.exp ...
gdb compile failed, Dwarf Error: wrong version in compilation unit header \
  (is 153, should be 2, 3, 4 or 5) [in module \
  build/gdb/testsuite/outputs/gdb.dwarf2/dw2-error/.tmp/dw2-error]
...
and similar for target board cc-with-debug-names.

Be less verbose by adding "quiet" to the compile flags.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-04-24  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/dw2-error.exp: Add quiet to compile flags.