fft_test: fix operator<< output

cast function pointers to void*; this matches other tests and fixes a
-Wmicrosoft-cast warning:

test\fft_test.cc(85,59): warning: implicit conversion between
pointer-to-function and pointer-to-object is a Microsoft extension
[-Wmicrosoft-cast]
  return os << "fft_arg { n:" << test_arg.n << " fft:" << test_arg.fft << " }";

output on Linux goes from:
  Correct/0  # GetParam() = fft_arg { n:2 fft:1 }
to:
  Correct/0  # GetParam() = fft_arg { n:2 fft:0x5636c6ce6960 }

Change-Id: I146c5cefcd3a7511146e6b1991f74af7d3f72852
diff --git a/test/fft_test.cc b/test/fft_test.cc
index 7fce0f8..04d047d 100644
--- a/test/fft_test.cc
+++ b/test/fft_test.cc
@@ -82,7 +82,8 @@
 };
 
 std::ostream &operator<<(std::ostream &os, const FFTTestArg &test_arg) {
-  return os << "fft_arg { n:" << test_arg.n << " fft:" << test_arg.fft << " }";
+  return os << "fft_arg { n:" << test_arg.n
+            << " fft:" << reinterpret_cast<const void *>(test_arg.fft) << " }";
 }
 
 class FFT2DTest : public ::testing::TestWithParam<FFTTestArg> {
@@ -171,8 +172,8 @@
 };
 
 std::ostream &operator<<(std::ostream &os, const IFFTTestArg &test_arg) {
-  return os << "ifft_arg { n:" << test_arg.n << " fft:" << test_arg.ifft
-            << " }";
+  return os << "ifft_arg { n:" << test_arg.n
+            << " fft:" << reinterpret_cast<const void *>(test_arg.ifft) << " }";
 }
 
 class IFFT2DTest : public ::testing::TestWithParam<IFFTTestArg> {