Pull in the riscv64 uapi headers for riscv64 builds.

Test: treehugger
Change-Id: I0c41a1a5b9805057609c68a49f67e6e24d4a0572
diff --git a/libc/Android.bp b/libc/Android.bp
index d985c81..e59fac2 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -1921,6 +1921,9 @@
         arm64: {
             export_system_include_dirs: ["kernel/uapi/asm-arm64"],
         },
+        riscv64: {
+            export_system_include_dirs: ["kernel/uapi/asm-riscv"],
+        },
         x86: {
             export_system_include_dirs: ["kernel/uapi/asm-x86"],
         },