rts.cabal.in: Move `extra-source-files` so it is valid

rts/rts.cabal.in

@@ -4,6 +4,11 @@ version: 1.0.2
 license: BSD-3-Clause
 maintainer: glasgow-haskell-users@haskell.org
 build-type: Simple
+
+extra-source-files:
+  -- This file needs to be in the package but shouldn't be compiled on its own.
+  xxhash.c
+
 flag libm
   default: @CabalHaveLibm@
 flag librt
@@ -571,10 +576,6 @@ library
                -- I wish we had wildcards..., this would be:
                -- *.c hooks/**/*.c sm/**/*.c eventlog/**/*.c linker/**/*.c
 
-    extra-source-files:
-               -- This file needs to be in the package but shouldn't be compiled on its own.
-               xxhash.c
-
     if os(windows)
        c-sources: win32/AsyncMIO.c
                   win32/AsyncWinIO.c