libcurl: re-enable on non-MMU platforms

In 9229b82d ('libcurl: needs MMU'),
the libcurl package was disabled on non-MMU systems, due to the usage
of the fork() function in the library.

However, a deeper inspection reveals that fork() is only used in the
implementation of NTLM, an obscure, undocumented, Microsoft specific
authentication method that apparently isn't common anymore. See
http://curl.haxx.se/docs/manpage.html#--ntlm

.

Therefore, this commit re-enables libcurl on non-MMU systems by
explicitly disabling the NTLM support. If someone ever needs NTLM
support in Buildroot's libcurl package, it will always be time to add
a libcurl sub-option to enable it.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>