Guard falloc.h inclusion to avoid build breaks

Summary: Depending on the order of include paths and versions of various headers we may end up in a situation where we'll encounter a build break caused by redefinition of constants. gcc-4.9-glibc-2.20 header update to include/bits/fcntl-linux.h introduced the definitions of FALLOC_FL_* constants. However, linux/falloc.h from kernel-headers also has FALLOC_FL_* constants defined. Therefore during the compilation we'll get "previously defined" errors.

Test Plan:
Both in the environment where the build break manifests (to make sure that the change fixed the problem) and in the environment where everything builds fine (to make sure that there are no regressions):

make clean
make -j 32

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D52821

util/posix_logger.h

@@ -16,9 +16,13 @@
 #include "port/sys_time.h"
 #include <time.h>
 #include <fcntl.h>
+
 #ifdef OS_LINUX
+#ifndef FALLOC_FL_KEEP_SIZE
 #include <linux/falloc.h>
 #endif
+#endif
+
 #include "rocksdb/env.h"
 #include "util/iostats_context_imp.h"
 #include <atomic>