[compiler-rt] Add missing #else clause to fix the build on NetBSD.

An #elif SANITIZER_SOLARIS clause was removed in D120048, but it also
removed the #else clause for the error fallback, causing the build to
fail on NetBSD.

Differential Revision: https://reviews.llvm.org/D146316

(cherry picked from commit 8510cf9)

Author: fcambus

compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cpp

@@ -583,6 +583,7 @@ static void GetTls(uptr *addr, uptr *size) {
       *addr = (uptr)tcb->tcb_dtv[1];
     }
   }
+#else
 #error "Unknown OS"
 #endif
 }