scripts: coccinelle: add fork as a reserved name

`fork()` is defined by POSIX and for creating a new child process.

It is the preferred method of doing so on systems with virtual memory and
the symbol should not be used outside of the context defined by the
POSIX standard.

https://pubs.opengroup.org/onlinepubs/9699919799/functions/fork.html

Signed-off-by: Chris Friedt <[email protected]>

Author: cfriedt

scripts/coccinelle/symbols.txt

@@ -49,6 +49,7 @@ fileno5
 floor
 fmod
 fopen
+fork
 fprintf
 fputc1
 fputs1