[gcc r11-4698] libstdc++: Ensure std::lock_guard is declared

[Jonathan Wakely]

https://gcc.gnu.org/g:e1276e334298251d73303999d2adc688abbfc856

commit r11-4698-ge1276e334298251d73303999d2adc688abbfc856
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Nov 3 21:56:44 2020 +0000

    libstdc++: Ensure std::lock_guard is declared
    
    libstdc++-v3/ChangeLog:
    
            * include/std/syncstream: Include <bits/std_mutex.h>
            unconditionally.

Diff:
---
 libstdc++-v3/include/std/syncstream | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/std/syncstream b/libstdc++-v3/include/std/syncstream
index 5e0864ec54a..9d1db0cf286 100644
--- a/libstdc++-v3/include/std/syncstream
+++ b/libstdc++-v3/include/std/syncstream
@@ -44,10 +44,7 @@
 #include <bits/allocator.h>
 #include <bits/functexcept.h>
 #include <bits/functional_hash.h>
-
-#if _GLIBCXX_HAS_GTHREADS
-# include <bits/std_mutex.h>
-#endif
+#include <bits/std_mutex.h>
 
 namespace std _GLIBCXX_VISIBILITY(default)
 {