common/zstd_compression: Remove #pragma once directive from source file

Introduced in 2d755221e8. This is only
necessary within header files.

src/common/zstd_compression.cpp

@@ -2,8 +2,6 @@
 // Licensed under GPLv2 or any later version
 // Refer to the license.txt file included.
 
-#pragma once
-
 #include <algorithm>
 #include <zstd.h>