Add PCRE2_EXTRA_NEVER_CALLOUT flag to pcre2.h.generic (#510)

diff --git a/src/pcre2.h.generic b/src/pcre2.h.generic
index 46d0b4e..45da899 100644
--- a/src/pcre2.h.generic
+++ b/src/pcre2.h.generic
@@ -161,6 +161,7 @@
 #define PCRE2_EXTRA_ASCII_DIGIT              0x00001000u  /* C */
 #define PCRE2_EXTRA_PYTHON_OCTAL             0x00002000u  /* C */
 #define PCRE2_EXTRA_NO_BS0                   0x00004000u  /* C */
+#define PCRE2_EXTRA_NEVER_CALLOUT            0x00008000u  /* C */
 
 /* These are for pcre2_jit_compile(). */