[gdb/testsuite] Be quiet about untested dtrace-prob.exp

[gdb/testsuite] Be quiet about untested dtrace-prob.exp

When running gdb.base/dtrace-probe.exp, I get this on stdout/stderr:
...
Running src/gdb/testsuite/gdb.base/dtrace-probe.exp ...
gdb compile failed, ld: error in \
  build/gdb/testsuite/outputs/gdb.base/dtrace-probe/dtrace-probe.o\
  (.eh_frame); no .eh_frame_hdr table will be created
ld: crt1.o: in function `_start':
start.S:110: undefined reference to `main'
ld: build/gdb/testsuite/outputs/gdb.base/dtrace-probe/dtrace-probe-p.o:\
  (.SUNW_dof+0x88): undefined reference to `main'
ld: build/gdb/testsuite/outputs/gdb.base/dtrace-probe/dtrace-probe-p.o:\
  (.SUNW_dof+0xb8): undefined reference to `main'
collect2: error: ld returned 1 exit status

                === gdb Summary ===

nr of untested testcases         1
...

There is no reason to be this verbose about the failure to compile.

Fix this by using quiet as additional option to gdb_compile in
dtrace_build_usdt_test_program.  Note that the error message still occurs in
gdb.log.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-02-19  Tom de Vries  <tdevries@suse.de>

	* lib/dtrace.exp (dtrace_build_usdt_test_program): Use quiet as
	gdb_compile option.