add explicit BR for the gcc compiler

... as instructed at:

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

units.spec

@@ -1,7 +1,7 @@
 Summary: A utility for converting amounts from one unit to another
 Name: units
 Version: 2.16
-Release: 3%{?dist}
+Release: 4%{?dist}
 Source: https://ftp.gnu.org/gnu/units/%{name}-%{version}.tar.gz
 URL: https://www.gnu.org/software/units/units.html
 License: GPLv3+
@@ -9,6 +9,7 @@ Requires(post): /sbin/install-info
 Requires(preun): /sbin/install-info
 BuildRequires: automake
 BuildRequires: bison
+BuildRequires: gcc
 BuildRequires: ncurses-devel
 BuildRequires: python3
 BuildRequires: readline-devel
@@ -63,6 +64,9 @@ fi
 %{_mandir}/man1/*
 
 %changelog
+* Mon Feb 19 2018 Kamil Dudka <kdudka@redhat.com> - 2.16-4
+- add explicit BR for the gcc compiler
+
 * Fri Feb 09 2018 Fedora Release Engineering <releng@fedoraproject.org> - 2.16-3
 - Rebuilt for https://fedoraproject.org/wiki/Fedora_28_Mass_Rebuild