cstring: wrap const-generic string functions in parentheses to bypass glibc macros

When using glibc 2.43+ and defining _GNU_SOURCE, the header <string.h>
implicitly enables C23 source features. This activates the new type-generic
__glibc_const_generic macros for core functions like strchr, memchr,
strrchr, strstr, and strpbrk.

Consequently, if these functions are passed a `const char*` or `const void*`,
the macros enforce a `const` return type, even when compiling in older C standards.

Because our test functions act as wrappers that intentionally return mutable
pointers from const arguments, compiling this file was generating warnings.

Wrap the function identifiers in parentheses to suppresses preprocessor
macro expansion, forcing the legacy behavior.

Author: nunoplopes

rules/cstring/src.c

@@ -13,27 +13,27 @@ int f3(const void *s1, const void *s2, size_t n) { return memcmp(s1, s2, n); }
 
 void *f4(void *dst, const void *src, size_t n) { return memmove(dst, src, n); }
 
-char *f5(const char *a0, int a1) { return strchr(a0, a1); }
+char *f5(const char *a0, int a1) { return (strchr)(a0, a1); }
 
 size_t f7(const char *a0) { return strlen(a0); }
 
 int f8(const char *a0, const char *a1) { return strcmp(a0, a1); }
 
 int f9(const char *a0, const char *a1, size_t a2) { return strncmp(a0, a1, a2); }
 
-void *f10(const void *a0, int a1, size_t a2) { return memchr(a0, a1, a2); }
+void *f10(const void *a0, int a1, size_t a2) { return (memchr)(a0, a1, a2); }
 
-char *f11(const char *a0, int a1) { return strrchr(a0, a1); }
+char *f11(const char *a0, int a1) { return (strrchr)(a0, a1); }
 
 char *f15(const char *a0) { return strdup(a0); }
 
 size_t f16(const char *a0, const char *a1) { return strcspn(a0, a1); }
 
 size_t f17(const char *a0, const char *a1) { return strspn(a0, a1); }
 
-char *f18(const char *a0, const char *a1) { return strstr(a0, a1); }
+char *f18(const char *a0, const char *a1) { return (strstr)(a0, a1); }
 
-char *f21(const char *a0, const char *a1) { return strpbrk(a0, a1); }
+char *f21(const char *a0, const char *a1) { return (strpbrk)(a0, a1); }
 
 #if defined(__linux__)
 void *f24(const void *a0, int a1, size_t a2) { return memrchr(a0, a1, a2); }