add explicit BR for the gcc compiler

... as instructed at:

https://fedoraproject.org/wiki/Packaging:C_and_C%2B%2B

zsh.spec

@@ -1,7 +1,7 @@
 Summary: Powerful interactive shell
 Name: zsh
 Version: 5.4.2
-Release: 5%{?dist}
+Release: 6%{?dist}
 License: MIT
 URL: http://zsh.sourceforge.net/
 # FIXME: use .xz once available
@@ -19,6 +19,7 @@ Patch1:  0001-zsh-5.4.2-funcnest.patch
 BuildRequires: autoconf
 BuildRequires: coreutils
 BuildRequires: gawk
+BuildRequires: gcc
 BuildRequires: gdbm-devel
 BuildRequires: libcap-devel
 BuildRequires: ncurses-devel
@@ -170,6 +171,9 @@ fi
 %doc Doc/*.html
 
 %changelog
+* Mon Feb 19 2018 Kamil Dudka <kdudka@redhat.com> - 5.4.2-6
+- add explicit BR for the gcc compiler
+
 * Fri Feb 09 2018 Fedora Release Engineering <releng@fedoraproject.org> - 5.4.2-5
 - Rebuilt for https://fedoraproject.org/wiki/Fedora_28_Mass_Rebuild