manual: Fix a syntax error.

The opening parenthesis for function arguments in an @deftypefun need
to be separated from the function name.  This isn't just a matter of
the GNU coding style---it causes the "(void" (in this case) to be
rendered as a part of the function name, causing a visual defect, and
also results in a warning to the following effect during `make pdf':

  Warning: unbalanced parentheses in @def...)

	* manual/platform.texi (__riscv_flush_icache): Fix @deftypefun
	syntax.

(cherry picked from commit 16efad51)