tc: fix bpf compilation with old glibc

Error was:
f_bpf.o: In function `bpf_parse_opt':
f_bpf.c:(.text+0x88f): undefined reference to `secure_getenv'
m_bpf.o: In function `parse_bpf':
m_bpf.c:(.text+0x587): undefined reference to `secure_getenv'
collect2: error: ld returned 1 exit status

There is no special reason to use the secure version of getenv, thus let's
simply use getenv().

CC: Daniel Borkmann <daniel@iogearbox.net>
Fixes: 88eea539 ("tc: {f,m}_bpf: allow to retrieve uds path from env")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Tested-by: Yegor Yefremov <yegorslists@googlemail.com>

examples/bpf/bpf_agent.c

@@ -154,7 +154,7 @@ static void bpf_map_get_from_env(int *tfd)
 		memset(key, 0, sizeof(key));
 		snprintf(key, sizeof(key), "BPF_MAP%d", i);
 
-		val = secure_getenv(key);
+		val = getenv(key);
 		assert(val != NULL);
 
 		tfd[i] = atoi(val);

tc/f_bpf.c

@@ -122,7 +122,7 @@ opt_bpf:
 
 			NEXT_ARG();
 			if (ebpf) {
-				bpf_uds_name = secure_getenv(BPF_ENV_UDS);
+				bpf_uds_name = getenv(BPF_ENV_UDS);
 				bpf_obj = *argv;
 				NEXT_ARG();
 

tc/m_bpf.c

@@ -105,7 +105,7 @@ opt_bpf:
 
 			NEXT_ARG();
 			if (ebpf) {
-				bpf_uds_name = secure_getenv(BPF_ENV_UDS);
+				bpf_uds_name = getenv(BPF_ENV_UDS);
 				bpf_obj = *argv;
 				NEXT_ARG();