dt-binding: fix compilation error of the example in qcom,gcc.yaml

Running `make dt_binging_check`, gives the error:

DTC     Documentation/devicetree/bindings/clock/qcom,gcc.example.dt.yaml
Error: Documentation/devicetree/bindings/clock/qcom,gcc.example.dts:111.28-29 syntax error
FATAL ERROR: Unable to parse input tree

This is because the last example uses the macro RPM_SMD_XO_CLK_SRC which
is defined in qcom,rpmcc.h but the include of this header is missing.
Add the include to fix the error.

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Link: https://lkml.kernel.org/r/20200122134639.11735-1-dafna.hirschfeld@collabora.com
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

Documentation/devicetree/bindings/clock/qcom,gcc.yaml

@@ -220,6 +220,7 @@ examples:
 
   # Example of MSM8998 GCC:
   - |
+    #include <dt-bindings/clock/qcom,rpmcc.h>
     clock-controller@100000 {
       compatible = "qcom,gcc-msm8998";
       #clock-cells = <1>;