libc/Android.bp: make libc_headers visible to llvm-libc

llvm-libc will reuse parts of the existing system headers for types that have
implications on the ABI.

Fixes:
    error: external/llvm-libc/Android.bp:3:1: module "llvmlibc" variant
    "android_recovery_riscv64_static": depends on //bionic/libc:libc_headers
    which is not visible to this module You may need to add
    "//external/llvm-libc" to its visibility

Test: TH
Bug: 321313756
Change-Id: I33b93d8e1617a480249641dc2cdf4bbee9f9e19d
diff --git a/libc/Android.bp b/libc/Android.bp
index 5afed1a..396a069 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -1897,6 +1897,7 @@
         "//external/gwp_asan",
         "//external/jemalloc_new",
         "//external/libunwind_llvm",
+        "//external/llvm-libc",
         "//external/scudo",
         "//system/core/property_service/libpropertyinfoparser",
         "//system/extras/toolchain-extras",