Use standard header locations for poll.h and signal.h

sys/poll.h and sys/signal.h just redirect to the standardized
location for these headers, poll.h and signal.h.

On musl libc, including the incorrect header path results in a
warning, and since breakpad is built with -Werror, this is an error.

In exception_handler.cc, signal.h is already included earlier, so
we can drop the sys/signal.h include.

Bug: google-breakpad:631
Change-Id: If36d326453e3267d38a5b92ed1301f828e46befe
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/2097344
Reviewed-by: Mike Frysinger <vapier@chromium.org>

src/client/linux/handler/exception_handler.cc

@@ -78,7 +78,6 @@
 #include <sys/wait.h>
 #include <unistd.h>
 
-#include <sys/signal.h>
 #include <sys/ucontext.h>
 #include <sys/user.h>
 #include <ucontext.h>

src/client/linux/handler/exception_handler_unittest.cc

@@ -27,12 +27,12 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+#include <poll.h>
 #include <pthread.h>
 #include <stdint.h>
 #include <unistd.h>
 #include <signal.h>
 #include <sys/mman.h>
-#include <sys/poll.h>
 #include <sys/socket.h>
 #include <sys/uio.h>
 #include <sys/wait.h>

src/client/linux/minidump_writer/linux_ptrace_dumper_unittest.cc

@@ -36,13 +36,13 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <limits.h>
+#include <poll.h>
 #include <unistd.h>
 #include <signal.h>
 #include <stdint.h>
 #include <string.h>
 #include <sys/mman.h>
 #include <sys/prctl.h>
-#include <sys/poll.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 

src/client/linux/minidump_writer/minidump_writer_unittest.cc

@@ -28,7 +28,7 @@
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include <fcntl.h>
-#include <sys/poll.h>
+#include <poll.h>
 #include <sys/stat.h>
 #include <sys/syscall.h>
 #include <sys/types.h>