Fix potentially uninitialized struct used on MSVC

Summary:
MSVC warns this might be used but uninitialized. Force
the struct to be zero initialized.

Reviewed By: yfeldblum

Differential Revision: D5585320

fbshipit-source-id: 9454a2a4a66c7689f42b1eb211dc57f5d3b88fea

folly/SharedMutex.h

@@ -850,6 +850,7 @@ class SharedMutexImpl {
                             WaitContext& ctx) {
 #ifdef RUSAGE_THREAD
     struct rusage usage;
+    std::memset(&usage, 0, sizeof(usage));
     long before = -1;
 #endif
     for (uint32_t yieldCount = 0; yieldCount < kMaxSoftYieldCount;
@@ -1005,6 +1006,7 @@ class SharedMutexImpl {
 
 #ifdef RUSAGE_THREAD
     struct rusage usage;
+    std::memset(&usage, 0, sizeof(usage));
     long before = -1;
 #endif
     for (uint32_t yieldCount = 0; yieldCount < kMaxSoftYieldCount;