pfring: copy cflags over from src/

This should cause the following warning to error out:

runmode-pfring.c: In function 'OldParsePfringConfig':
runmode-pfring.c:118:17: error: implicit declaration of function 'StringParseUnt16'; did you mean 'StringParseInt16'? [-Wimplicit-function-declaration]
  118 |             if (StringParseUnt16(&pfconf->threads, 10, 0, threadsstr) < 0) {
      |                 ^~~~~~~~~~~~~~~~
      |                 StringParseInt16

Ticket: #8271

plugins/pfring/Makefile.am

@@ -8,5 +8,20 @@ noinst_HEADERS = \
 	runmode-pfring.h \
 	source-pfring.h
 
+# default CFLAGS
+AM_CFLAGS = ${OPTIMIZATION_CFLAGS} ${GCC_CFLAGS} ${CLANG_CFLAGS}            \
+    ${SECCFLAGS} ${PCAP_CFLAGS} ${EXTRA_CFLAGS}                                                         \
+    -Wall -Wno-unused-parameter -Wmissing-prototypes -Wmissing-declarations \
+    -Wstrict-prototypes -Wwrite-strings -Wbad-function-cast                 \
+    -Wformat-security -Wno-format-nonliteral -Wmissing-format-attribute     \
+    -funsigned-char
+
+# different flags for different cases
+if DEBUG
+AM_CFLAGS += -ggdb -O0
+endif
+
+AM_LDFLAGS = ${SECLDFLAGS}
+
 install-exec-hook:
 	cd $(DESTDIR)$(pkglibdir) && $(RM) $(pkglib_LTLIBRARIES)