Fix building libprocinfo against musl

Musl libc provides a definition of gettid().

Bug: 190084016
Test: m USE_HOST_MUSL=true libpcap
Change-Id: Ie8c0eb85c640ab7e0e5cfe6bcabf8bcd532e63ac
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);