Load theme bug

If you have `php-mode-coding-style` set and you run `load-theme`, it
calls `php-mode-custom-coding-style-set` from
`custom-theme-recalc-variable`.  This happens regardless of what type
of buffer you are in, so if you are in a buffer whose major mode is
not based on `cc-mode`, you can get an error like this:

```
Debugger entered--Lisp error: (error "Buffer  *temp* is not a CC Mode buffer (c-set-style)")
  signal(error ("Buffer  *temp* is not a CC Mode buffer (c-set-style)"))
  error("Buffer %s is not a CC Mode buffer (c-set-style)" " *temp*")
  c-set-style("symfony2")
  php-enable-symfony2-coding-style()
  php-mode-custom-coding-style-set(php-mode-coding-style symfony2)
  custom-theme-recalc-variable(php-mode-coding-style)
  enable-theme(user)
  enable-theme(solarized-dark)
  load-theme(solarized-dark)
  eval((load-theme (quote solarized-dark)) nil)
  eval-last-sexp-1(nil)
  eval-last-sexp(nil)
  call-interactively(eval-last-sexp nil nil)
```
 This patch makes `php-mode-custom-coding-style-set` check to make
 sure it's in a `php-mode` buffer.