elinks.spec: add build dependency on gcc-c++

It is for some reason expected by autoconf:

[...]
checking for variadic macros... yes
checking how to run the C++ preprocessor... /lib/cpp
configure: error: in `/builddir/build/BUILD/elinks-0.15.0':
configure: error: C++ preprocessor "/lib/cpp" fails sanity check
See `config.log' for more details
RPM build errors:
error: Bad exit status from /var/tmp/rpm-tmp.md0nhQ (%build)

elinks.spec

@@ -10,7 +10,7 @@ Source2:   elinks.conf
 BuildRequires: automake
 BuildRequires: bzip2-devel
 BuildRequires: expat-devel
-BuildRequires: gcc
+BuildRequires: gcc-c++
 BuildRequires: gpm-devel
 BuildRequires: krb5-devel
 BuildRequires: libidn2-devel