modem-helpers-qmi: include string.h for memset()

This patch fixes the following compiler warning:

mm-modem-helpers-qmi.c:568:5: error: implicitly declaring library function 'memset' with type 'void *(void *, int, unsigned int)' [-Werror,-Wimplicit-function-declaration]
    memset (extended_qmi_lte_bands, 0, extended_qmi_lte_bands_size * sizeof (guint64));
    ^