lxc/start.c: Fix legacy PR_{G,S}ET_NO_NEW_PRIVS handling

lxc/start.c: Fix legacy PR_{G,S}ET_NO_NEW_PRIVS handling

The configure checks for these use AC_CHECK_DECLS, which define the symbol
to 0 if not available - So adjust the code to match.  From the autoconf
manual:

https://www.gnu.org/software/autoconf/manual/autoconf-2.65/html_node/Generic-Declarations.html)

For each of the symbols (comma-separated list), define HAVE_DECL_symbol (in
all capitals) to ‘1’ if symbol is declared, otherwise to ‘0’.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>