[GWP-ASan] Include <unistd.h> for sysconf(_SC_PAGESIZE) (#155261)

This fixes build failures on Fuchsia that started with #153860

GitOrigin-RevId: c05567842b73a2286ba6d84cac09693d41f0c838

tests/basic.cpp

diff --git a/tests/basic.cpp b/tests/basic.cpp
index 7d36a2e..ec2cacc 100644
--- a/tests/basic.cpp
+++ b/tests/basic.cpp
@@ -8,6 +8,8 @@
 
 #include "gwp_asan/tests/harness.h"
 
+#include <unistd.h>
+
 TEST_F(CustomGuardedPoolAllocator, BasicAllocation) {
   InitNumSlots(1);
   void *Ptr = GPA.allocate(1);

tests/never_allocated.cpp

diff --git a/tests/never_allocated.cpp b/tests/never_allocated.cpp
index 37a4b38..6e1ee47 100644
--- a/tests/never_allocated.cpp
+++ b/tests/never_allocated.cpp
@@ -12,6 +12,8 @@
 #include "gwp_asan/crash_handler.h"
 #include "gwp_asan/tests/harness.h"
 
+#include <unistd.h>
+
 TEST_P(BacktraceGuardedPoolAllocatorDeathTest, NeverAllocated) {
   size_t PageSize = sysconf(_SC_PAGESIZE);