kallsyms: fix build without execinfo

Some libc's like musl libc don't provide execinfo.h since it's not part of
POSIX.  In order to fix compilation on musl, only include execinfo.h if
available (HAVE_BACKTRACE_SUPPORT)

This was discovered with c104c16 ("Kunit to check the longest symbol
length") which starts to include linux/kallsyms.h with Alpine Linux'
configs.

Link: https://lkml.kernel.org/r/20250622014608.448718-1-fossdd@pwned.life
Fixes: c104c16 ("Kunit to check the longest symbol length")
Signed-off-by: Achill Gilgenast <fossdd@pwned.life>
Cc: Luis Henriques <luis@igalia.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: fossdd

tools/include/linux/kallsyms.h

@@ -18,6 +18,7 @@ static inline const char *kallsyms_lookup(unsigned long addr,
 	return NULL;
 }
 
+#ifdef HAVE_BACKTRACE_SUPPORT
 #include <execinfo.h>
 #include <stdlib.h>
 static inline void print_ip_sym(const char *loglvl, unsigned long ip)
@@ -30,5 +31,8 @@ static inline void print_ip_sym(const char *loglvl, unsigned long ip)
 
 	free(name);
 }
+#else
+static inline void print_ip_sym(const char *loglvl, unsigned long ip) {}
+#endif
 
 #endif