cgen: fix for function-like macro '_BIONIC_AVAILABILITY_GUARD' which is not defined in older termux releases (#26345)

Author: Krchi

vlib/v/gen/c/cheaders.v

@@ -315,7 +315,10 @@ typedef int (*qsort_callback_func)(const void*, const void*);
 #undef __attribute__
 #endif
 #ifdef __TERMUX__
-#if defined __BIONIC_AVAILABILITY_GUARD && __BIONIC_AVAILABILITY_GUARD(28)
+#if !defined(__BIONIC_AVAILABILITY_GUARD)
+	#define __BIONIC_AVAILABILITY_GUARD(api_level) 0
+#endif
+#if __BIONIC_AVAILABILITY_GUARD(28)
 #else
 void * aligned_alloc(size_t alignment, size_t size) { return malloc(size); }
 #endif