cpu_used_firstpass_test: fix -Wfloat-conversion warning

use the std::abs(double) overload to avoid:

test/cpu_used_firstpass_test.cc: In member function ‘void
{anonymous}::CpuUsedFirstpassTest::DoTest()’:
test/cpu_used_firstpass_test.cc:87:30: warning: conversion from ‘double’
to ‘int’ may change value [-Wfloat-conversion]
   87 |     psnr_diff = abs(ref_psnr - GetAveragePsnr());
      |                     ~~~~~~~~~^~~~~~~~~~~~~~~~~~

Change-Id: I0fd9a44041ea022bac5af3b0e47b34c2fc3cf0d9
diff --git a/test/cpu_used_firstpass_test.cc b/test/cpu_used_firstpass_test.cc
index c53db6e..cfffcd7a 100644
--- a/test/cpu_used_firstpass_test.cc
+++ b/test/cpu_used_firstpass_test.cc
@@ -9,6 +9,8 @@
  * PATENTS file, you can obtain it at www.aomedia.org/license/patent.
  */
 
+#include <cstdlib>
+
 #include "test/codec_factory.h"
 #include "test/encode_test_driver.h"
 #include "test/i420_video_source.h"
@@ -84,7 +86,7 @@
     first_pass_cpu_used_ = GET_PARAM(1);
     if (first_pass_cpu_used_ == second_pass_cpu_used_) return;
     ASSERT_NO_FATAL_FAILURE(RunLoop(&video));
-    psnr_diff = abs(ref_psnr - GetAveragePsnr());
+    psnr_diff = std::abs(ref_psnr - GetAveragePsnr());
     EXPECT_LT(psnr_diff, GetPsnrDiffThreshold())
         << "first pass cpu used = " << first_pass_cpu_used_
         << ", second pass cpu used = " << second_pass_cpu_used_;