Fix building libprocinfo against musl am: a4a14a3a9c

Original change: https://android-review.googlesource.com/c/platform/system/libprocinfo/+/1970019

Change-Id: Ie3a9b685fbf83224a9663cd853efff5005133c1d
diff --git a/process_test.cpp b/process_test.cpp
index 58a1546..0f25f92 100644
--- a/process_test.cpp
+++ b/process_test.cpp
@@ -35,7 +35,7 @@
 
 using namespace std::chrono_literals;
 
-#if !defined(__BIONIC__)
+#if defined(__GLIBC__)
 #include <syscall.h>
 static pid_t gettid() {
   return syscall(__NR_gettid);