Fix BPF_FD_JUST_USE_INT macro

Bug: 173167302
Test: atest BpfMapTest
Change-Id: I70b58e23cc5b04a187de8d55ddc3f2b367718b88
diff --git a/libbpf_android/include/bpf/BpfUtils.h b/libbpf_android/include/bpf/BpfUtils.h
index 428606a..310a889 100644
--- a/libbpf_android/include/bpf/BpfUtils.h
+++ b/libbpf_android/include/bpf/BpfUtils.h
@@ -29,7 +29,7 @@
 
 #ifdef BPF_FD_JUST_USE_INT
   #define BPF_FD_TYPE int
-  #define BPF_FD_TO_INT(x) (x)
+  #define BPF_FD_TO_INT(x) static_cast<__u32>(x)
 #else
   #include <android-base/unique_fd.h>
   #define BPF_FD_TYPE base::unique_fd&