util, iwyu: Add missed header

[hebasto]

This PR amends #34669 and fixes the CI.

[DrahtBot]

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Type	Reviewers
ACK	brunoerg, pinheadmz

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[brunoerg]

ACK d28179b

[pinheadmz]

ACK d28179b

should fix failure seen here https://github.com/bitcoin/bitcoin/actions/runs/25186108957/job/73843745290

>>> Fixing #includes in '/home/admin/actions-runner/_work/_temp/src/util/feefrac.h'
IWYU edited 1 files on your behalf.

Formatting src/util/feefrac.h
+ git --no-pager diff --exit-code
diff --git a/src/util/feefrac.h b/src/util/feefrac.h
index 702f5d4..f9c9a0a 100644
--- a/src/util/feefrac.h
+++ b/src/util/feefrac.h
@@ -9,6 +9,7 @@
 #include <util/overflow.h>
 
 #include <compare>
+#include <concepts>
 #include <cstdint>
 #include <span>
 #include <utility>
+ echo '^^^ ⚠️ Failure generated from IWYU'
^^^ ⚠️ Failure generated from IWYU
+ false
Command '['docker', 'exec', '--env', 'DANGER_RUN_CI_ON_HOST=1', 'f09d47524926eddf61f6a9dfa158aa864b8c907aab458b3cb1ce9caad3d8028f', '/home/admin/actions-runner/_work/_temp/ci/test/03_test_script.sh']' returned non-zero exit status 1.