common/lz4_compression: Remove #pragma once directive from the cpp file

Introduced within 798d76f4c7018174e58702fb06a042dc8c84f0be, this only
really has an effect within header files.

Silences a -Wpragma-once-outside-header warning with clang.

src/common/lz4_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 <lz4hc.h>