Fix compilation on AIX 7.3

On AIX, compilation fails saying

    src/http.c: In function 'op_http_conn_start_tls':
    src/http.c:1944:5: warning: ISO C forbids nested functions [-Wpedantic]
     1944 |     int                ip_len;
	  |     ^~~
    In file included from /usr/include/netinet/tcp.h:115,
		     from src/http.c:345:
    src/http.c:1944:24: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
     1944 |     int                ip_len;
    src/http.c:1944:24: error: expected expression before '.' token
    src/http.c:1949:5: error: 'ip_ff' undeclared (first use in this function); did you mean 'ip_fv'?
     1949 |     ip_len=0;

because `/usr/include/netinet/ip-h` contains

    #define  ip_len  ip_ff.ip_flen

The obvious solution os to rename the int variable to something else.