baselibc: Fix fflush duplicate error

This tries to fix error:
Linking review/bin/targets/nordic_pca10056-auracast_usb/app/@apache-mynewt-nimble/apps/auracast_usb/auracast_usb.elf
Error: Arm GNU Toolchain arm-none-eabi/13.2 Rel1/bin/../lib/gcc/arm-none-eabi/13.2.1/../../../../arm-none-eabi/bin/ld.exe: Arm GNU Toolchain arm-none-eabi/13.2 Rel1/bin/../lib/gcc/arm-none-
eabi/13.2.1/thumb/v7e-m/nofp\libg.a(libc_a-fflush.o): in function `fflush':
fflush.c:(.text.fflush+0x0): multiple definition of `fflush'; review/bin/targets/nordic_pca10056-auracast_usb/app/@apache-mynewt-core/libc/baselibc/@apache-mynewt-core_libc_baselibc.a(mynewt.o):review/repos/apache-mynewt-core/libc/baselibc/src/mynewt.c:54: first defined here

fflush is not used anywhere in the code but for some reason gcc 12 and 13 complains about
duplicate. When baselibc version of fflush is removed code compiles and links without
problem and final elf does not have fflush from libc_a-fflush.o.

Since baselib.c fflush version is practically empty making it normal static
inline does not breaks anything and linker stops failing.

libc/baselibc/include/stdio.h

@@ -97,9 +97,15 @@ __extern_inline int putchar(int c)
 #define getc(f) fgetc(f)
 #define getchar() fgetc(stdin)
 
-__extern_inline int fflush(FILE *stream)
+/*
+ * GCC 12 and later complained about duplicate symbols when
+ * __extern_inline was used instead of static inline even though
+ * fflush was never put in final elf. This approach may be extended
+ * to other functions with __extern_inline in the future.
+ */
+static inline int fflush(FILE *stream)
 {
-	return 0;
+    return 0;
 }
 
 __extern int printf(const char *, ...);