Add missing 'BuildRequires: gcc'

A C compiler is necessary to build Toolbx [1].  GCC is being pulled in
by the other BuildRequires, but it's good to explicitly list it since
GCC isn't part of the default buildroot since Fedora 29 [2] and will
prevent the compiler from unexpectedly changing to Clang.

There's no need to do a build just for this.

[1] Upstream commit c8aaed52c547e24e
    https://github.com/containers/toolbox/commit/c8aaed52c547e24e
    https://github.com/containers/toolbox/pull/923

[2] https://fedoraproject.org/wiki/Changes/Remove_GCC_from_BuildRoot

toolbox.spec

@@ -23,6 +23,7 @@ Patch101:      toolbox-Make-the-build-flags-match-Fedora-s-gobuild.patch
 Patch102:      toolbox-Make-the-build-flags-match-Fedora-s-gobuild-for-PPC64.patch
 Patch103:      toolbox-cmd-root-Work-around-Cobra-1.1.2-s-handling-of-usage.patch
 
+BuildRequires: gcc
 BuildRequires: go-md2man
 BuildRequires: golang >= 1.13
 %if ! 0%{?rhel}