use size_t to better represent thread id across platforms

include/spdlog/details/os.h

@@ -32,11 +32,11 @@
 #  define WIN32_LEAN_AND_MEAN
 # endif
 # include <Windows.h>
-#elif __linux__ 
+#elif __linux__
 #include <sys/syscall.h> //Use gettid() syscall under linux to get thread id
 #include <unistd.h>
 #else
-#include <pthread.h> 
+#include <pthread.h>
 #endif
 
 #include "../common.h"
@@ -172,14 +172,14 @@ inline int utc_minutes_offset(const std::tm& tm = details::os::localtime())
 }
 
 //Return current thread id as 64 bit integer
-inline uint64_t thread_id()
+inline size_t thread_id()
 {
 
 #ifdef _WIN32
     return ::GetCurrentThreadId();
 #elif __linux__
     return  (uint64_t) syscall(SYS_gettid);
-#else     
+#else
     return (uint64_t) pthread_self();
 #endif