[libc][NFC] Add fenv and string headers to x86_64 headers list.

GitOrigin-RevId: 2bf8be79b10cc41064bdbe3d001c48669027ccfe
diff --git a/config/linux/x86_64/headers.txt b/config/linux/x86_64/headers.txt
index 7baf61c..55504c1 100644
--- a/config/linux/x86_64/headers.txt
+++ b/config/linux/x86_64/headers.txt
@@ -2,11 +2,13 @@
     libc.include.assert_h
     libc.include.ctype
     libc.include.errno
+    libc.include.fenv
     libc.include.inttypes
     libc.include.math
     libc.include.signal
     libc.include.stdio
     libc.include.stdlib
+    libc.include.string
     libc.include.sys_mman
     libc.include.sys_syscall
     libc.include.threads