Fix build by fixing misleading indentation

A new warning in GCC was causing the build to fail with build-snapshot as it builds with -Werror:

  CC       src/mounts.lo
src/mounts.c: In function ‘rc_delete_btrfs_subvolume_at’:
src/mounts.c:514:7: error: this ‘if’ clause does not guard... [-Werror=misleading-indentation]
       if (saved_errno == ENOENT)
       ^~
src/mounts.c:517:9: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the ‘if’
         g_set_error (error, RC_ERROR, error_code,
         ^~~~~~~~~~~
cc1: all warnings being treated as errors
make[1]: *** [Makefile:1771: src/mounts.lo] Error 1

The code was correct, but the indentation was indeed misleading.