smp_call.h defines SMP-related structures and declares
SMP-related functions. These codes are only valid when
RT_USING_SMP is defined in the bsp, which means we have
to use RT_USING_SMP for judgment in every place where
this file is included, such as following:
```c
```
Now move the judgment of RT_USING_SMP directly into
smp_call.h, so that it's simpler to include the header
file as:
```c
```
Signed-off-by: Chen Wang <unicorn_wang@outlook.com>
724b2f1297