examples/c/usdt: fix a compilation error

Fix 'libbpf: failed to find BTF info for global/extern symbol' erro due
to an uninitialized global variables, which causes problems on Ubuntu
20.04.6 with clang 10.

I was able to compile on a newer ubuntu with a newer version of clang.
However, on a development module (nVidia Orin AGX) running Ubuntu
20.04.6 LTS with clang version 10.0.0-4ubuntu1, it is not possible to
use apt to update clang to a new version. Downloading the source code of
a higher version of clang for compilation is cumbersome.

So I hope that with the change, the project code can be easily compiled
and used in earlier os.

Signed-off-by: Liu Lingze <luiyanbing@foxmail.com>

Author: GorilaMond

examples/c/usdt.bpf.c

@@ -5,7 +5,7 @@
 #include <bpf/bpf_tracing.h>
 #include <bpf/usdt.bpf.h>
 
-pid_t my_pid;
+pid_t my_pid = 0;
 
 SEC("usdt/libc.so.6:libc:setjmp")
 int BPF_USDT(usdt_auto_attach, void *arg1, int arg2, void *arg3)