libutil/util_libc.c: Move remove_whitespace() to util_str_rm_whitespace()

Relocate remove_whitespace() from zdsfs to libutil/util_str.c, renaming
it to util_str_rm_whitespace() to make the helper available
project-wide.

Acked-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Jan Höppner <hoeppner@linux.ibm.com>
Signed-off-by: Jan Polensky <japo@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

include/lib/util_str.h

@@ -0,0 +1,17 @@
+/**
+ * @defgroup util_str_h util_str: String functions
+ * @{
+ * @brief Manipulate and work with strings
+ *
+ * Copyright IBM Corp. 2025
+ *
+ * s390-tools is free software; you can redistribute it and/or modify
+ * it under the terms of the MIT license. See LICENSE for details.
+ */
+
+#ifndef LIB_UTIL_STR_H
+#define LIB_UTIL_STR_H
+
+void util_str_rm_whitespace(const char *src, char *dest);
+
+#endif /** LIB_UTIL_STR_H @} */