[libc] Fix the CMake var name of the list of public headers.

GitOrigin-RevId: e958d491570633b7ae5ef5cbc83be43cc0e4c0fe
diff --git a/config/linux/aarch64/headers.txt b/config/linux/aarch64/headers.txt
index 6aa6681..f784070 100644
--- a/config/linux/aarch64/headers.txt
+++ b/config/linux/aarch64/headers.txt
@@ -1,4 +1,4 @@
-set(PUBLIC_HEADERS
+set(TARGET_PUBLIC_HEADERS
     libc.include.errno
     libc.include.math
     libc.include.stdlib