trace: remove invalid semicolon after if clause

trace: remove invalid semicolon after if clause

The commit 6f6f99a38132 ("trace: add non-hierarchical function_graph option"),
in https://android.googlesource.com/kernel/common.git/+/android-3.18,
added invalid semicolon after if clause, so it causes following
build error on gcc 6, so this patch removes the invalid seimicolon.

  kernel/trace/trace_output.c: In function 'trace_graph_ret_raw':
  kernel/trace/trace_output.c:976:2: error: this 'if' clause does not guard... [-Werror=misleading-indentation]
    if (!trace_seq_printf(&iter->seq, "%lx %lld %lld %ld %d\n",
    ^~
  kernel/trace/trace_output.c:982:3: note: ...this statement, but the latter is misleadingly indented as if it is guarded by the 'if'
     return TRACE_TYPE_PARTIAL_LINE;
     ^~~~~~

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>