BUILD: syscalls: remove improper inline statement in front of syscalls

Trying to build with an old gcc and glibc revealed that we must not
state "inline" in our _syscall* definitions since it's already present
in the declaration making use of the _syscall* macros.
diff --git a/include/common/splice.h b/include/common/splice.h
index a776b9b..ea29b16 100644
--- a/include/common/splice.h
+++ b/include/common/splice.h
@@ -62,7 +62,7 @@
 #define __NR_splice             313
 #endif /* __NR_splice */
 
-static _syscall6(int, splice, int, fdin, loff_t *, off_in, int, fdout, loff_t *, off_out, size_t, len, unsigned long, flags);
+static inline _syscall6(int, splice, int, fdin, loff_t *, off_in, int, fdout, loff_t *, off_out, size_t, len, unsigned long, flags);
 #endif /* VSYSCALL */
 
 #else