PR #2086: Include  <signal.h> for SIG* constants

Imported from GitHub PR https://github.com/abseil/abseil-cpp/pull/2086

The test uses `SIGABRT` on most of the systems, so explicitly include `<signal.h>` to ensure it is available, and not rely on it being implicitly pulled in by other includes.
Merge 65079f812861d29caf3273b85f28593accde9858 into ad568ab75f7464eb232855bd0f0a625aa1b784f0

Merging this change closes #2086

COPYBARA_INTEGRATE_REVIEW=https://github.com/abseil/abseil-cpp/pull/2086 from pinotree:test-include-signal.h 65079f812861d29caf3273b85f28593accde9858
PiperOrigin-RevId: 936962686
Change-Id: I738c40de2702b4c59b7210c4816751d2cd9b1e13

absl/log/internal/test_helpers.cc

diff --git a/absl/log/internal/test_helpers.cc b/absl/log/internal/test_helpers.cc
index bfcc967..63e9deb 100644
--- a/absl/log/internal/test_helpers.cc
+++ b/absl/log/internal/test_helpers.cc
@@ -18,6 +18,10 @@
 #include <zircon/syscalls.h>
 #endif
 
+#if defined(ABSL_HAVE_ALARM)
+#include <signal.h>
+#endif
+
 #include "gtest/gtest.h"
 #include "absl/base/config.h"
 #include "absl/base/log_severity.h"