journal/fsprg: set -Wno-pointer-arithm only for that file

Both gcc and clang issue a host of warnings about void pointers used in
arithmetic. The warning must be ignored in that file to avoid multiple
warnings.

Makefile.am used to set this for all libsystemd-journal-internal.a sources,
because there's no finer granularity for warnings. Let's just set it for
this one file.

Makefile.am

@@ -4556,10 +4556,8 @@ libsystemd_journal_internal_la_SOURCES += \
 libsystemd_journal_internal_la_LIBADD += \
 	$(GCRYPT_LIBS)
 
-# fsprg.c is a drop-in file using void pointer arithmetic
 libsystemd_journal_internal_la_CFLAGS += \
-	$(GCRYPT_CFLAGS) \
-	-Wno-pointer-arith
+	$(GCRYPT_CFLAGS)
 endif
 
 noinst_LTLIBRARIES += \

src/journal/fsprg.c

@@ -40,6 +40,9 @@
 #define RND_GEN_Q 0x02
 #define RND_GEN_X 0x03
 
+#pragma GCC diagnostic ignored "-Wpointer-arith"
+/* TODO: remove void* arithmetic and this work-around */
+
 /******************************************************************************/
 
 static void mpi_export(void *buf, size_t buflen, const gcry_mpi_t x) {