[compiler-rt][hwasan] Check for SANITIZER_POSIX before including sanitizer_posix.h

Rather than throwing an error. This way we can still use files like
hwasan_dynamic_shadow.cpp for other platforms without leading to a
preprocessor error.

Differential Revision: https://reviews.llvm.org/D106979

compiler-rt/lib/sanitizer_common/sanitizer_posix.h

@@ -20,10 +20,7 @@
 #include "sanitizer_platform_limits_posix.h"
 #include "sanitizer_platform_limits_solaris.h"
 
-#if !SANITIZER_POSIX
-// Make it hard to accidentally use any of functions declared in this file:
-#error This file should only be included on POSIX
-#endif
+#if SANITIZER_POSIX
 
 namespace __sanitizer {
 
@@ -126,4 +123,6 @@ void DecorateMapping(uptr addr, uptr size, const char *name);
 
 }  // namespace __sanitizer
 
+#endif  // SANITIZER_POSIX
+
 #endif  // SANITIZER_POSIX_H