[llvm-exegesis] Fix in SubprocessMemoryTest after commit adb01de

Make sure the TestCount const definition is guarded the same way
as the use of the constant.

This is an attempt to fix buildbot failures related to
-Wunused-const-variable.

Author: bjope

llvm/unittests/tools/llvm-exegesis/X86/SubprocessMemoryTest.cpp

@@ -20,15 +20,15 @@
 #include <unistd.h>
 #endif // __linux__
 
-// This needs to be updated anytime a test is added or removed from the test
-// suite.
-static constexpr const size_t TestCount = 4;
-
 namespace llvm {
 namespace exegesis {
 
 #if defined(__linux__) && !defined(__ANDROID__)
 
+// This needs to be updated anytime a test is added or removed from the test
+// suite.
+static constexpr const size_t TestCount = 4;
+
 class SubprocessMemoryTest : public X86TestBase {
 protected:
   int getSharedMemoryNumber(const unsigned TestNumber) {