scripts: coccinelle: add errno as a reserved name

errno is defined by the ISO C standard to be a modifiable lvalue of type
int, and **must not be explicitly declared**

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

scripts/coccinelle/symbols.txt

@@ -31,6 +31,7 @@ difftime64
 div
 erf
 erfc
+errno
 exit
 exp
 fabs