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

This fixes build failures on Fuchsia that started with #153860

Author: fabio-d

compiler-rt/lib/gwp_asan/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);

compiler-rt/lib/gwp_asan/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);