kcov: add missing dependency (#21932)

kcov CMakeLists.txt generates the "kcov" executable only if
certain dependencies are found. These dependencies are 
"libbfd", "libopcodes" and "libiberty", hence the dependency 
on binutils.

var/spack/repos/builtin/packages/kcov/package.py

@@ -19,6 +19,8 @@ class Kcov(CMakePackage):
     depends_on('cmake@2.8.4:', type='build')
     depends_on('zlib')
     depends_on('curl')
+    depends_on('elfutils')
+    depends_on('binutils +libiberty', type='link')
 
     def cmake_args(self):
         # Necessary at least on macOS, fixes linking error to LLDB