fatal error: 'uapi/linux/bpf.h' file not found

I am new to BPF and trying to run a simple hello_program as below.
```
#include
#define SEC(NAME) __attribute__((section(NAME), used))

SEC("tracepoint/syscalls/sys_enter_execve")
int bpf_prog(void *ctx) {
char msg[] = "Hello, BPF World!";
bpf_trace_printk(msg, sizeof(msg));
return 0;
}
char _license[] SEC("license") = "GPL";
```
Getting following error

```
clang -O2 -target bpf -c hello_world_kern.c -o hello_world_kern.o
hello_world_kern.c:1:10: fatal error: 'uapi/linux/bpf.h' file not found
#include
^~~~~~~~~~~~~~~~~~
1 error generated.
```
My linux version is
```
lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04.1 LTS
Release: 18.04
Codename: bionic

uname -r
4.15.0-46-generic
```

Begin with hello_world_kern.c and the clang command shown; investigate why the uapi/linux/bpf.h include cannot be resolved in the stated Ubuntu 18.04 and kernel environment. Done means the example compiles successfully and the required header setup is identified.

Written by the indexing model from the issue text.